Migrate martor guide to featherlight

[int-y1]

part of #2035 - this makes the martor guide dark mode friendly

• apply dmoj theme to martor guide
• for admin martor, give access to featherlight.css and featherlight.min.js
• set max-height: 85% to avoid issues with the admin site's z-index

screenshots

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.73%. Comparing base (fd7fb05) to head (2f5bed2).
Report is 56 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2391      +/-   ##
==========================================
+ Coverage   46.76%   47.73%   +0.97%     
==========================================
  Files         251      259       +8     
  Lines       13317    13706     +389     
==========================================
+ Hits         6228     6543     +315     
- Misses       7089     7163      +74     

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