feat: Unify slide upload and proposal

[jimfenton]

This removes the propose_session_slides page, URL, and view function and merges them into upload_session_slides. It adds a checkbox, visible to chairs, allowing them to propose, rather than automatically approve, slides they are submitting.

Fixes #7705
Fixes #7178

The tests still have separate test_upload_slides and test_propose_session_slides functions. Some of this may be redundant, but I thought it better to do possibly overtest. Perhaps the tests should be combined as well.

[codecov]

Codecov Report

Attention: Patch coverage is 97.22222% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.78%. Comparing base (c7f6bde) to head (6879762).
Report is 23 commits behind head on main.

Files	Patch %	Lines
ietf/meeting/views.py	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7787      +/-   ##
==========================================
- Coverage   88.78%   88.78%   -0.01%     
==========================================
  Files         296      296              
  Lines       41320    41322       +2     
==========================================
+ Hits        36687    36688       +1     
- Misses       4633     4634       +1     

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