[META] Polish backend editing UX to the max

Problem/Motivation

At the paragraphs workshop we identified that we can improve the backend editing UX a lot. No need to come up with something completely new yet.

Proposed resolution

Let's collect all the related issues to order and priorise them as part of this issue...

#2738645: Add a collapse / edit all button
#2658694: Move a nested Paragraph across fields and nesting
#2738715: Improve Paragraph collapsed/closed summary
#2737863: [META] Allow to add a paragraph between any item
#2577229: Display paragraphs as open when item count is low
#2668708: Spacing between fields on paras

Remaining tasks

User interface changes