Blocks: Try to move filtering blocks until editor mounts

[gziolo]

Description

Resolves: #4848.

WIP

In the ideal case we should be able to support the following:

• register block override
• register block
• register block override
• initialize editor and let users write their post
• register block (and apply all previously registered overrides)
• unregister block
• register block override and apply to all matching blocks
• remove override and update matching blocks

How has this been tested?

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[gziolo]

I don't plan to work further on this one since we are going to change the way how blocks can be registered after we agree on #13693. Depending on the final shape of blocks registration the solution proposed here might be completely different.