Notes: Add autosaving drafts and extending session (no UX changes yet)

[kevinrobinson]

Who is this PR for?

all educators, particularly during remote meetings with multiple apps or windows

What problem does this PR fix?

If someone is running a meeting remotely (eg, MTSS meeting), they start writing a note but then switch to other tabs or browser windows (eg, for videochat). If they don't remember to circle back, they may lose what they had timed when the session times out. #2810 aims to make that scenario more visible.

Similarly, if they prep for the remote meeting and load up the first student profile beforehand, but then that page sits for ~25 minutes before the meeting start, they might start typing the note, switch away to videochat, but have the session expire because there's no server interaction for so long.

What does this PR do?

Debounces changes to draft notes, and sends them up to the server. For now, the server writes these to EventNoteDrafts but doesn't otherwise use them. This PR will keep the session alive in this scenario, and the next step will be the UI for discovering unsaved drafts and 'publishing' them as notes. There's no UX changes in this PR.

The 'draft' concept is a compromise between making it so folks don't have to think about saving at all (a la docs), and concerns that folks have about the potential for harm with immediately publishing as they type (eg, when typing through how to phrase privacy-sensitive issues).

Separately, this PR factors out a RestrictedTextRedacter that different models use to redact sensitive text in #as_json by default, and uses it in new EventNoteDraft model as well, with tests all around.

Checklists

Which features or pages does this PR touch?

• Student profile, taking notes

Does this PR use tests to help verify we can deploy these changes quickly and confidently?

• Included specs for changes
• Improved specs for existing code in need of better test coverage
• Manual testing made more sense here

[kevinrobinson]

selfie