Add styling for spoilers with HTML <details> tag

[quantum5]

Preview:

[Xyene]

LGTM

[codecov-commenter]

Codecov Report

Merging #1774 (f45c818) into master (df3029c) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1774      +/-   ##
==========================================
+ Coverage   46.19%   46.23%   +0.04%     
==========================================
  Files         223      223              
  Lines       12676    12829     +153     
==========================================
+ Hits         5856     5932      +76     
- Misses       6820     6897      +77     

Impacted Files	Coverage Δ
judge/views/problem.py	30.76% <0.00%> (+5.81%)	⬆️

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