Simplify blocks edit page

[ajpal]

This is a mostly-no-op change with a small bug fix.
It seems like the intention was to have initializeCodeMirrorForJson be a general purpose function, but at the same time, its code was really intertwined with the specific blocks edit page use case. I decided it would be cleaner to have the blocks edit page own all of its own logic, and just call initializeCodeMirror directly.

Along the way, I found that the onChange callback was not being called, since #27138 changed initializeCodeMirror to accept an options object, and the call in intializeCodeMirrorForJson was not changed. This didn't seem to really cause any issues, since the callback was still called on blur, but this PR fixes that so the preview and generated code live-update as you change the config.

Testing story

Reviewer Checklist:

• Tests provide adequate coverage
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[maddiedierker]

❤️ thank you for this refactor!