* feat: Show bluesheets using Attended tables (#6898)

* feat: Allow users to add themselves to session attendance (#6454)

* chore: Correct copyright year

* fix: Address review comments

* fix: Don't try to generate empty bluesheets

* refactor: Complete rewrite of bluesheet.html

* refactor: Fill in a few gaps, close a few holes

- Rename the live "bluesheet" to "attendance", add some explanatory text.
- Add attendance links in materials view and pre-finalized proceedings view.
- Don't allow users to add themselves after the corrections cutoff date.

* fix: Report file-save errors to caller

* fix: Address review comments

* fix: typo

* refactor: if instead of except; refactor gently

* refactor: Rearrange logic a little, add comment

* style: Black

* refactor: auto_now_add->default to allow override

* refactor: jsonschema to validate API payload

* feat: Handle new API data format

Not yet tested except that it falls back when the old
format is used.

* test: Split test into deprecated/new version

Have not yet touched the new version

* style: Black

* test: Test new add_session_attendees API

* fix: Fix bug uncovered by test

* refactor: Refactor affiliation lookup a bit

* fix: Order bluesheet by Attended.time

* refactor: Move helpers from views.py to utils.py

* test: Test that finalize calls generate_bluesheets

* test: test_bluesheet_data()

* fix: Clean up merge

* fix: Remove debug statement

* chore: comments

* refactor: Renumber migrations

---------

Co-authored-by: Paul Selkirk <paul@painless-security.com>

chore: Merge main into feat/sess-apis

(API changed on their end)

Removed some type checking rather than debugging it

feat: add Meetecho to upload/remove slides; refactor upload_session_slides

fix: consistent URL for slides; test fixup

feat: send Meetecho slides updates in doc UI

fix: better Meetecho API "order" management

Don't let errors in the MeetEcho slides API interfere with
the ability to modify slides for a session.

feat: Only send slides notifications near session time

fix: request_timeout is optional; fix test bug

* fix: save slides before sending notification

* style: fix indentation

It's not a bug, it's a flourish!