Add Support for Reusable Blocks

[SiobhyB]

Reusable blocks allow users to quickly re-use blocks, without needing to remember specific formatting or settings. This can be a great time-saver. Mobile users especially may appreciate being able to quickly select a block that they know is formatted in a way that looks good across devices. Reusable blocks aren't currently accessible from the app, however. It'd be great to add them!

This originally came up in a one-star app review from a user who seems to heavily rely on reusable blocks when setting up their posts/pages:

Pretty useless for me. All of my pop ups...reusable blocks etc are unsupported.

This feature is split in several parts:

• Create new reusable blocks: Add Support to create Reusable Blocks #2574
• Use existing reusable blocks in the app: Add Support to Insert Existing Reusable Blocks from the Editor #2575
• Edit existing reusable blocks: Allow reusable blocks to be edited in the editor #2597
• Manage existing reusable blocks: Manage existing reusable blocks #2598

[hypest]

Will move this ticket to the gutenberg-mobile repo as it's an editor issue, and cross-platform as well.