Add support for inner block attributes

[yscik]

Changes proposed in this Pull Request

• Render block based on the course structure data
• Add a blocks attribute to the Outline block, containing attributes for it's inner blocks
  • This is indexed by lesson/modules id, like lesson-13. This way, if things are modified/reordered outside the block editor, the renderer can still use the style & block-level settings set for that lesson/module.
• Merge with existing blocks in the editor when loading the course structure data from the backend. (Preserve their attributes.)

Testing instructions

• Create a course, add modules and lessons, save
• View course, check that those modules and lessons are displayed in the frontend.
  --
• The attributes stuff is mostly infrastructure, actual usage for block styling will be in separate PRs.

Screenshot / Video

Teaser:

[jom]

I'll hold off approving until the dependent PRs get merged, but in principle this looks good! ✅

[jom]

Looks good!