Use sentence case headings consistently

[Xyene]

Once you notice they're not consistent with the rest of the site, you can't unsee it.

[codecov-io]

Codecov Report

Merging #1567 into master will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1567      +/-   ##
==========================================
- Coverage   46.15%   46.02%   -0.14%     
==========================================
  Files         211      212       +1     
  Lines       12087    12133      +46     
==========================================
+ Hits         5579     5584       +5     
- Misses       6508     6549      +41     

Impacted Files	Coverage Δ
judge/views/api/api_v2.py	47.46% <0.00%> (-1.56%)	⬇️
judge/utils/infinite_paginator.py	63.91% <0.00%> (-1.30%)	⬇️
judge/views/comment.py	39.69% <0.00%> (-0.62%)	⬇️
judge/views/submission.py	32.66% <0.00%> (-0.25%)	⬇️
judge/views/user.py	38.92% <0.00%> (ø)
judge/management/commands/move_user_content.py	0.00% <0.00%> (ø)

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