fix: Always label agenda session headers with day in meeting time zone

[jennifer-richards]

Fixes #3323 (the part left in this comment)

[codecov]

Codecov Report

Merging #5282 (e637169) into main (0ec1264) will decrease coverage by 0.01%.
The diff coverage is 94.52%.

@@            Coverage Diff             @@
##             main    #5282      +/-   ##
==========================================
- Coverage   88.58%   88.57%   -0.01%     
==========================================
  Files         295      294       -1     
  Lines       40114    40071      -43     
==========================================
- Hits        35534    35494      -40     
+ Misses       4580     4577       -3     

Impacted Files	Coverage Δ
ietf/iesg/urls.py	100.00% <ø> (ø)
ietf/meeting/models.py	86.03% <ø> (-0.05%)	⬇️
ietf/meeting/urls.py	81.25% <ø> (ø)
ietf/iesg/views.py	92.53% <85.18%> (-0.71%)	⬇️
ietf/doc/mails.py	96.20% <100.00%> (ø)
ietf/doc/utils_search.py	80.58% <100.00%> (ø)
ietf/group/views.py	90.95% <100.00%> (+0.09%)	⬆️
ietf/ietfauth/views.py	92.12% <100.00%> (+1.09%)	⬆️
ietf/meeting/helpers.py	89.84% <100.00%> (-0.07%)	⬇️
ietf/meeting/views.py	91.12% <100.00%> (-0.01%)	⬇️
... and 5 more

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