Create a grid config page for controlling all defaults at once

WIP.

Going to see if I can have some help with the UX around this.

Todo:

• Allow the form to be saved.
• Allow new defaults to be added (i.e. the 'add' functionality).
• Switch to using the metatag object's form field instead of hardcoding a textfield.
• Add dynamic search to hide certain columns.
• Make sure the form is accessible (see https://www.mediacurrent.com/blog/accessible-names-label-all-things-part-2).

The idea is to replace the giant spreadsheet I came up with for planning the defaults, having it all on one page makes it easier to see them all at a glance.

This changes the form to use the metatag's form method, moves the tag's description into the field's 'title' attribute, and adds a column to indicate the config's state.

This adds a save button and fixes the token handling so the form doesn't throw token validation errors. Note: it doesn't actually save yet ;-)

The save operation now works! :-)

This adds an option to enable or disable the configuration, and a way of adding new entity/bundle configs.

This removes the '#states' logic because the form was getting a bit messy.

Right now it adds a separate page that looks like this:

All of the meta tags scroll out to the right, with each configuration a separate row.

At the bottom of the page is the following:

Anyone have any thoughts on what the page should be called? "Grid" seems a little too unhelpful.

@Dave Reid: That's entirely reasonable. I also anticipate possible problems with the form submission when several submodules are enabled and with lots of entity types because of the number of fields that would be submitted.

@Dave Reid: What do you think of "metatag_alt_ui", or "metatag_advanced_ui"?

Hey there :) I've tweaked the stories a little to focus on the default configurations.

@danigrrl: Thanks for your help. I'll get back to this soon.

Maybe it could be a submodule that just replaces the existing config page? So you'd turn on the module and the existing "Defaults" page would be replaced by the grid. Hrm..

One thing this would have to watch for is the max_input_vars variable, maybe there should be a hook_requirements() check to see if it would even be feasible to save the form based upon what it's set to.

A submodule.

Needs tests.

Fixed form saving, added some messages to hook_requirements, and added tests to ensure that toggling a config's status will work.

This reworked the structure so that there are separate columns for each tag group, rather than each tag being in a separate column.

This adds all for each config that were present on the non-grid display, but I've had to disable the tests as they need to be redone.

I think the better approach would be to show all tags horizontally and then have a search (like the Module Filter module) to limit the display.

This goes back to having one column for each tag.