feat: new meeting registration implementation

[rpcross]

New implementation of meeting registrations. Moved from stats.MeetingRegistration to meeting.Registration and meeting.RegistrationTickets. Also new API and management command, migrate_registrations, to copy MeetingRegistration records to Registration.

This is just the first step in a series. The management command can be called from cron regularly to populate the new table, Registration, from the old. Future commits will migrate all the existing code that uses MeetingRegistration to using Registration. Once that is complete the Registration system can be changed to use the new API, stop running the migration, then remove the old table and API.

This PR replaces #7724

[codecov]

Codecov Report

Attention: Patch coverage is 84.89583% with 29 lines in your changes missing coverage. Please review.

Project coverage is 88.78%. Comparing base (1b4afc9) to head (76736d4).
Report is 49 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/api/views.py	73.61%	19 Missing ⚠️
ietf/meeting/models.py	83.33%	4 Missing ⚠️
ietf/meeting/utils.py	92.45%	4 Missing ⚠️
ietf/meeting/tasks.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8408      +/-   ##
==========================================
- Coverage   88.80%   88.78%   -0.02%     
==========================================
  Files         314      314              
  Lines       41213    41434     +221     
==========================================
+ Hits        36601    36789     +188     
- Misses       4612     4645      +33     

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[jennifer-richards]

I still have a few things to think about, but wanted to get some feedback to you. I like the direction. We'll need to create a feat branch to receive the series of PRs before merging to main. Maybe feat/registration would be a good name?

[jennifer-richards]

Looking good. For the record, we discussed out-of-band whether the initial=True branch of the migration utility would be better as a Django migration or done manually, but I think it's better to keep it as it's done here.

I think the names.json needs an update. Would also be good to combine the migrations (i.e., remove them and regenerate)

One other comment inline.