fix: improve chat log rendering

[NGPixel]

• Replace colon-style emojis with system emojis
• Highlight user mentions
• Format blockquotes
• Hide broken inline images
• Fix spacing of paragraphs
• Add bs5 color reference scss variables

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This PR also removes .json from the rsync assets exclude list, as its needed to fetch chat logs and is very small anyway.

[codecov]

Codecov Report

Merging #4686 (0b249dd) into main (69c0f4d) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4686      +/-   ##
==========================================
- Coverage   88.41%   88.40%   -0.01%     
==========================================
  Files         296      296              
  Lines       39730    39730              
==========================================
- Hits        35126    35123       -3     
- Misses       4604     4607       +3     

Impacted Files	Coverage Δ
ietf/nomcom/utils.py	91.26% <100.00%> (-0.25%)	⬇️
ietf/group/utils.py	93.72% <0.00%> (-0.90%)	⬇️
ietf/doc/views_search.py	89.02% <0.00%> (-0.21%)	⬇️
ietf/utils/pipe.py	86.95% <0.00%> (+4.34%)	⬆️

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