fix: versioned href for slides URL

[jennifer-richards]

Sends a versioned URL constructed by DocumentInfo.get_href() instead of get_versionless_href(). This should avoid cache-related issues when Meetecho updates their slides. It also fixes a bug (which, afaik, has not been encountered) where the latest slides would always be shown, even if a specific version was selected.

The comment about why get_versionless_href() was used appears to be stale - I think it was fixed in #7172, which was committed before any of this ever hit the main branch to begin with. I checked all the uses and do not see any situations where it a slides document (or related SessionPresentation) is changed/saved after a SlidesManager() method is called.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.89%. Comparing base (c7f6bde) to head (5de8765).
Report is 94 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8040      +/-   ##
==========================================
+ Coverage   88.78%   88.89%   +0.10%     
==========================================
  Files         296      304       +8     
  Lines       41320    41302      -18     
==========================================
+ Hits        36687    36715      +28     
+ Misses       4633     4587      -46     

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[rjsparks]

cc: @alexamirante (for awareness)