feat: share precomputed agenda data with agenda.ics

[jennifer-richards]

Speeds up the agenda.ics endpoints to near instant. Assuming we keep the caches warm as intended, it will also speed up all agenda page loads.

• Refactors the agenda_ical() and its helpers to use the same precomputed data that is supplied to the agenda-neue frontend.
• Adds agenda caching for all meetings, not just the current meeting, with a week-scale cache lifetime for non-current meetings.
• Changes the ical links to use versionless agenda hrefs so they'll always point to the current revision.
• Adds a task to populate/refresh the cache for all meetings so it will always be warm.
• Renames agenda_data_refresh to agenda_data_refresh_task (keeps/deprecates old task until we deploy and update our config to use the new name)
• Fixes caching for get_href / get_versionless_href mixes data between these calls #10355

I propose that we continue refreshing the agenda data for the "current" meeting every 5 minutes but only weekly for other meetings. That was assumed in choosing the new constants in settings.py. This does not add a mechanism for invalidating the cache dynamically if the agenda is changed. A change like that will need a manual cache refresh if waiting for the next regular refresh is not acceptable.

Note: The cache is kept in memcache. Data for each meeting is 200-300 kB, so it fits well within memcache's size limit. Restarting memcached will lose cached data, though. I think this is acceptable because memcached restarts are rare and the cache will recover on its own as requests come in, usually with at most one slow request per meeting.

[codecov]

Codecov Report

❌ Patch coverage is 83.56164% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.37%. Comparing base (a174f43) to head (4740c3c).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/meeting/views.py	82.20%	21 Missing ⚠️
ietf/meeting/tasks.py	85.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10356      +/-   ##
==========================================
- Coverage   88.38%   88.37%   -0.02%     
==========================================
  Files         325      325              
  Lines       43525    43638     +113     
==========================================
+ Hits        38470    38563      +93     
- Misses       5055     5075      +20     

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