Implement user display name override

[quantum5]

Fixes #1745.

[codecov-commenter]

Codecov Report

Merging #1747 (beb20d8) into master (e86c114) will increase coverage by 0.02%.
The diff coverage is 46.15%.

@@            Coverage Diff             @@
##           master    #1747      +/-   ##
==========================================
+ Coverage   46.49%   46.52%   +0.02%     
==========================================
  Files         220      221       +1     
  Lines       12454    12461       +7     
==========================================
+ Hits         5791     5797       +6     
- Misses       6663     6664       +1     

Impacted Files	Coverage Δ
judge/admin/profile.py	62.22% <ø> (ø)
judge/views/contests.py	29.96% <ø> (ø)
judge/views/select2.py	50.74% <0.00%> (ø)
judge/views/submission.py	32.17% <0.00%> (ø)
judge/views/user.py	38.53% <ø> (ø)
judge/models/profile.py	86.84% <66.66%> (-0.33%)	⬇️
judge/migrations/0122_username_display_override.py	100.00% <100.00%> (ø)

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[kiritofeng]

Doesn't override problem author, nor https://github.com/DMOJ/online-judge/blob/master/templates/ticket/message.html#L5

[quantum5]

@kiritofeng fixed tickets, not sure what you mean about author, since it appears to work on the problem page.

[kiritofeng]

LGTM; I guess the thing about problem authors might've been some caching nonsense.

[Ninjaclasher]

Code looks good, but do we want to support filtering by the override in the select2 widget and profile admin?