feat: Add tracking checked-in to MeetingRegistration and the registration API.

[rpcross]

Add support for MeetingRegistration.checkedin field and fix #3672

[codecov]

Codecov Report

Merging #4280 (3f6078f) into main (6ba9114) will increase coverage by 0.21%.
The diff coverage is 96.87%.

@@            Coverage Diff             @@
##             main    #4280      +/-   ##
==========================================
+ Coverage   88.20%   88.42%   +0.21%     
==========================================
  Files         295      294       -1     
  Lines       39279    39225      -54     
==========================================
+ Hits        34645    34683      +38     
+ Misses       4634     4542      -92     

Impacted Files	Coverage Δ
ietf/group/urls.py	100.00% <ø> (ø)
ietf/group/views.py	90.90% <96.00%> (+2.11%)	⬆️
ietf/doc/views_doc.py	90.53% <100.00%> (+0.04%)	⬆️
ietf/stats/models.py	85.36% <100.00%> (+0.36%)	⬆️
ietf/stats/utils.py	54.23% <100.00%> (+0.78%)	⬆️
ietf/doc/views_draft.py	91.90% <0.00%> (-0.09%)	⬇️
ietf/meeting/views.py	90.78% <0.00%> (-0.04%)	⬇️
ietf/api/urls.py	100.00% <0.00%> (ø)
ietf/utils/markdown.py	100.00% <0.00%> (ø)
ietf/doc/views_bofreq.py	99.14% <0.00%> (+0.01%)	⬆️
... and 9 more

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