[META] Improve UX for block layout administration page

Problem/Motivation

We're using Drupal since Drupal 6 and yesterday a team member raised the point that the block layout administration page (/admin/structure/block) hasn't really been changed / improved since Drupal 6, while other areas improved a lot. ;D

We never thought about that before, as we just knew it that way, but while the block administration is still a substantial part of Drupal and the block concept on site-wide level is still very important, that part didn't seem to get a lot of UX / SX attention.

This META issue should help to pick up existing and new issues to improve this page and make a plan for the small and large steps required.

Steps to reproduce

Contexts where you might not be happy with the current UX / SX:
Use the block layout administration page (/admin/structure/block)...

...in large projects with many blocks required

• No JS-filtering of many blocks in region possible, like for example on the module list page (in-browser search required): #3273173: Allow to JS-filter blocks in regions on the block layout administration page
• Regions are not collapsible (for better overview): #3273174: Make regions collapsible on the block layout administration page
• Visibility conditions are not visible on this level, making it unclear what is visible on which page #3273175: Display active visibility conditions for blocks on the block layout administration page
• Block content can not be edited directly: #1956134: Provide helpful editing links on "admin/structure/block" for deriver blocks (menu, views, block content, etc.)
  #2407761: Add a way to edit block content on the block layout page
• Regions do not define high-level visibility conditions, which might be helpful if conditions are equal for all blocks and to hide regions completely: #3273176: Allow visibility conditions for regions
• ...?

... as new Drupal user / site owner:

• Block concept might be unclear without visualizing the block regions and "Demonstrate block regions" link hard to find: #1164748: Without first seeing the demonstration page, the list of possible regions makes no sense.
• Blocks can only be placed one by one
• Block contents can not be edited directly: #1956134: Provide helpful editing links on "admin/structure/block" for deriver blocks (menu, views, block content, etc.)
• Block content vs. other blocks logic might be hard to understand in general #2862564: Move Custom block library to Content
• UI for publishing/unpublishing block_content is still missing in contrast to other entity types: #2834546: UI for publishing/unpublishing block_content blocks
• Blocks should allow entering a multiline description: #1795268: Block metadata should use the standard "name" and "description" fields; follow content type form layout as the model
• Block (content) deletion is unclear: #1956496: Custom block contextual links should differentiate between editing/deleting the block instance and the custom block entity
• Configure block form is complicated: #2079037: Use two columns for the place block form
• In contrast to nodes, block contents have no "Preview" functionality: #213085: UMN Usability: Preview feature for blocks
• Place block filter only filters the name, expected to also filter category #2711735: Place block should also allow to filter by category
• Condition UI is not unified as UX could be improved:
  #2269881: [meta] Unifying condition UIs
  #1880168: Introduce top-level sections for all forms
  #2789241: Usability Issue - Block configuration form
  #2079037: Use two columns for the place block form
• Hard to administer on small devices: #2807831: Region select option is very small on narrow screens
  #2554167: Mobile Friendly?
• #2317981: Move block content edit and delete routes under admin/content/block to improve IA for editors and fix breadcrumbs
• #1919922: Improve DX around setting the theme when adding a (custom) block
• #2730605: Improve the SBX of Configuration in Block Module
• #2358971: Open the configure links on admin/structure/block in a modal
• #361918: Change "Page specific visibility settings" in block configuration to more meaningful language.
• #1164732: Users expected to see actual/sample text within block demonstration page (Add sample text to block demo page)
• #2087211: Move the 'Add custom block' local action to be in the place blocks sidebar
• #2087207: Only show the theme selector on the block form if more than one theme is enabled
• #2825413: When a block is being placed, selected region should be highlighted.
• #2085201: Use sensible block categories (rather than module-name categories)
• #2532054: Move the block layout page to Appearance
• ...?

Proposed resolution

Small, but very helpful improvements

(Make the entries above issue links)

Substantial rethinking / redesign

#954412: Explore alternative block interfaces goes even one step further and suggests to "rethink" the whole block administration page. As this will need some time, before doing that, we should pick the low-hanging fruits.

Remaining tasks

1. Look for already existing issues for the aspects above and link them in the issue summary or create new issues for them
2. Discuss and order the issues by relevancy and priority
3. Discuss to put this issue as parent issue into the listed issues
4. Get things done in the child issues
5. Have a great new Drupal Blocks experience! :)

User interface changes

API changes

Data model changes

Release notes snippet