feat(agenda): fallback link to plaintext version

[NGPixel]

Adds fallback links to loading screen and error screen:

Fixes #4655

[codecov]

Codecov Report

Merging #4663 (7775025) into main (d34e88d) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4663      +/-   ##
==========================================
- Coverage   88.43%   88.40%   -0.03%     
==========================================
  Files         296      296              
  Lines       39716    39730      +14     
==========================================
+ Hits        35122    35123       +1     
- Misses       4594     4607      +13     

Impacted Files	Coverage Δ
ietf/meeting/views.py	90.52% <100.00%> (ø)
ietf/utils/timezone.py	91.17% <100.00%> (+6.17%)	⬆️
ietf/utils/text.py	85.97% <0.00%> (-1.22%)	⬇️
ietf/group/utils.py	93.72% <0.00%> (-0.90%)	⬇️
ietf/doc/views_search.py	89.44% <0.00%> (-0.83%)	⬇️
ietf/nomcom/utils.py	90.77% <0.00%> (-0.49%)	⬇️
ietf/utils/mail.py	79.62% <0.00%> (-0.47%)	⬇️
ietf/submit/forms.py	82.70% <0.00%> (-0.18%)	⬇️

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[larseggert]

Nice fix. I just wish we wouldn't continue to build a ton of custom CSS instead of using b5. It will be a pain to move to newer bs versions.