Allow sorting of organization's user list

[int-y1]

Closes #1483. Makes #1533 redundant.

Allows sorting for the organization user list, and the class user list.

Also, clean up the css so that the headers are displayed as white.

[codecov-commenter]

Codecov Report

Merging #1977 (b5e81f3) into master (8faf2bf) will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1977      +/-   ##
==========================================
+ Coverage   46.51%   46.52%   +0.01%     
==========================================
  Files         236      236              
  Lines       13085    13093       +8     
==========================================
+ Hits         6086     6092       +6     
- Misses       6999     7001       +2     

Impacted Files	Coverage Δ
judge/views/organization.py	38.50% <66.66%> (+0.89%)	⬆️

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[Xyene]

LGTM, thanks!