fix: Use session name instead of timeslot name on agenda

[jennifer-richards]

Fixes #4457.

This uses the session name instead of the timeslot name to label special (type != "regular") sessions on the meeting agenda. The timeslot name is used for regular session headings ("Session I", "Session II", etc). This allows the agenda to be updated using the "Edit session" links from the schedule editor.

Changes the rendered agendas for some existing meetings. Includes a data migration to replace Session.name with TimeSlot.name for all non-'regular' sessions of IETF meetings except meetings 91-98. The meetings in that range had more descriptive names on the Session instances. I have not exhaustively compared meeting agendas, but all the agendas I examined looked the same or better with these updates.

A list of the session names that changed on my dev instance:
session-names.csv
The old session names are labeled session.name in the file. The new names are labeled timeslot.name.

[codecov]

Codecov Report

Merging #5086 (06d59b4) into main (bfe25fc) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 06d59b4 differs from pull request most recent head af8750a. Consider uploading reports for the commit af8750a to get more accurate results

@@            Coverage Diff             @@
##             main    #5086      +/-   ##
==========================================
- Coverage   88.50%   88.49%   -0.01%     
==========================================
  Files         296      296              
  Lines       40018    40020       +2     
==========================================
+ Hits        35416    35417       +1     
- Misses       4602     4603       +1     

Impacted Files	Coverage Δ
ietf/meeting/views.py	90.70% <100.00%> (+<0.01%)	⬆️
ietf/nomcom/utils.py	91.54% <0.00%> (-0.49%)	⬇️
ietf/utils/mail.py	79.85% <0.00%> (+0.23%)	⬆️

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