feat: Limit schedule generator by session purpose

[jennifer-richards]

Fixes #4921

Note that the schedule generator only works with "regular" type sessions. These are currently only used by three session purposes ("regular", "presentation", and "closed_meeting"):

>>> from ietf.name.models import *
>>> SessionPurposeName.objects.values_list('slug', 'timeslot_types')
<QuerySet [('none', []), ('regular', ['regular']), ('tutorial', ['other']), ('officehours', ['other']), ('coding', ['other']), ('admin', ['other', 'reg']), ('social', ['break', 'other']), ('plenary', ['plenary']), ('presentation', ['other', 'regular']), ('open_meeting', ['other']), ('closed_meeting', ['other', 'regular'])]>

The code in this PR will need minor tweaks to allow additional timeslot types if they become available.

[codecov]

Codecov Report

Merging #5055 (6f3c6d1) into main (570bd14) will decrease coverage by 0.02%.
The diff coverage is 93.84%.

@@            Coverage Diff             @@
##             main    #5055      +/-   ##
==========================================
- Coverage   88.47%   88.45%   -0.02%     
==========================================
  Files         296      296              
  Lines       39818    39856      +38     
==========================================
+ Hits        35230    35256      +26     
- Misses       4588     4600      +12     

Impacted Files	Coverage Δ
ietf/doc/views_draft.py	91.79% <50.00%> (-0.17%)	⬇️
ietf/secr/areas/views.py	37.66% <50.00%> (-0.50%)	⬇️
ietf/meeting/views.py	90.69% <98.00%> (+0.12%)	⬆️
ietf/doc/views_doc.py	91.17% <100.00%> (ø)
ietf/iesg/agenda.py	92.06% <100.00%> (+0.06%)	⬆️
ietf/utils/text.py	86.58% <100.00%> (+0.60%)	⬆️
ietf/meeting/templatetags/proceedings_filters.py	88.23% <0.00%> (-5.89%)	⬇️
ietf/utils/pipe.py	82.60% <0.00%> (-4.35%)	⬇️
ietf/meeting/models.py	85.91% <0.00%> (-0.59%)	⬇️
ietf/group/models.py	92.30% <0.00%> (-0.37%)	⬇️
... and 3 more

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