Add reusable option to inline block creation

Problem/Motivation

We are now able to create inline blocks from the Layout Builder but they are always set to non-reusable. It would be nice to have a checkbox to decide of the block should be reusable or not, similar to functionality in FPP for Drupal 7. This would allow editors to create reusable blocks while editing layouts for individual pieces of content, rather than having to jump to the "Block layout" section.

Proposed resolution

• Add a "Reusable" checkbox to inline_block plugins in Layout Builder
• If checked, it'll expose an "Admin title" field for naming the reusable block
• Once saved, it'll convert the underlying content block entity to reusable, and convert the inline_block plugin on the Section to a block_content plugin
• Allow editing the content block referenced by block_content plugins, and include a warning that let's editors know that their changes will be reflected globally

Completed tasks

@tbd

Remaining tasks and feedback

• Determine if this is something we should tackle in core
• UX Feedback from #2999491-88: Add reusable option to inline block creation: We felt that a checkbox on the block's edit form is the wrong place to change it from non-reusable to reusable. This is a change that cannot be undone. The usual way to handle that is not a warning in the description text for the checkbox: it is to use a confirmation form. There was strong consensus for this recommendation.
• UX Feedback from #2999491-88: Add reusable option to inline block creation: We already have an irreversible action for a block: deleting it. We should use the same pattern for both. That is, add an item to the contextual links. Selecting the link triggers a confirmation form in the off-canvas sidebar.
• UX Feedback from #2999491-88: Add reusable option to inline block creation: At the same time, review the text for removing a block (from the layout). Perhaps we should have different text depending on whether the block is reusable.
• UX Feedback from #2999491-88: Add reusable option to inline block creation: Having the confirmation form should make it easier to convey the implications for access control: see #38. Describe the other implications in terms that make sense to the content editor: the block will appear in the custom block library; it can be used in more than one place.
• UX Feedback from #2999491-88: Add reusable option to inline block creation: It will also be convenient to choose reusable or not when adding a new block. There are several options, and we do not have a strong recommendation. For example: two links, something like "Add reusable block" and "Add block for this page only". Or, after choosing "Create custom block", select reusable or not along with the block type.
• UX Feedback from #2999491-87: Add reusable option to inline block creation: Fix UX on status message.
• UX question from #2999491-38: Add reusable option to inline block creation: I just wonder if people will understand if they add a block and check this box the content they add via the block will be available for others to see regardless of the state of the entity. So if you add on page and that page is draft meaning most users can't see the page, the reusable blocks will be immediately available to other users who have access to use custom blocks.
• UX question from #2999491-40: Add reusable option to inline block creation: is it possible for a user to find themselves in a situation where the site permissions are setup in such a way where they can edit a Layout Override of an Entity and can add non-reusable custom blocks through LB, but can't create site-wide reusable blocks through the Custom Block UI? If that is possible, how would the additions in this patch handle that, would the user still be able to create reusable custom blocks via LB?
• UX question from #2999491-80: Add reusable option to inline block creation: For this patch I simply suggest to have discard changes not act on re-usable blocks and clearly state so.
• Conduct usability review (again), per #2999491-88: Add reusable option to inline block creation.
• Write tests

Follow-up issues

#3303306: Add an option to fork a reusable block to a non-reusable inline-block (and optionally delete the reusable block IF it's not used elsewhere:

...It might be a good idea to have a follow-up issue to decide how to choose when adding a new block.

User interface changes

• Adds checkbox and admin title field when editing inline_block in Layout Builder:
  
• Adds content block fields and warning when editing block_content in Layout Builder:
  

API changes

None.

Data model changes

None.

Per #2999491-88: Add reusable option to inline block creation:

It might be a good idea to have a follow-up issue to decide how to choose when adding a new block.

Release notes snippet

@todo