Add contest curator and testers

[kiritofeng]

Fixes #1556.

[codecov-commenter]

Codecov Report

Merging #1668 (7b841c6) into master (61145c5) will increase coverage by 0.15%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #1668      +/-   ##
==========================================
+ Coverage   46.25%   46.40%   +0.15%     
==========================================
  Files         215      216       +1     
  Lines       12282    12347      +65     
==========================================
+ Hits         5681     5730      +49     
- Misses       6601     6617      +16     

Impacted Files	Coverage Δ
judge/admin/contest.py	44.73% <0.00%> (ø)
.../migrations/0110_default_output_prefix_override.py	100.00% <ø> (ø)
judge/models/tests/util.py	97.65% <ø> (ø)
judge/views/api/api_v2.py	47.58% <0.00%> (-0.20%)	⬇️
judge/views/submission.py	32.66% <0.00%> (ø)
judge/views/contests.py	30.05% <10.00%> (-0.10%)	⬇️
...udge/migrations/0116_contest_curator_and_tester.py	100.00% <100.00%> (ø)
judge/models/contest.py	95.71% <100.00%> (+0.21%)	⬆️
judge/models/tests/test_contest.py	100.00% <100.00%> (ø)
... and 1 more

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[kiritofeng]

Obligatroy speling iz harrd

[kiritofeng]

Removed Contest.object.is_editor(...) entirely, and replaced all instances with ... in editor_ids instead.

[quantum5]

Nothing seems obviously broken to me.

[Ninjaclasher]

Otherwise LGTM