Add option to hide problem authors while in contest

[Ninjaclasher]

Refs #1032. (not sure why the issue was closed)

Addressing the issues that were faced in the previous attempt (#1033):

• With Add contest curator and testers #1668, we will assign the contest authors as assignees to prevent leaking problem authors.
• The API no longer shows problems that can only be accessed when in a contest, so that route won't leak authors either.

Have not tested the changes yet, so will create a draft PR for now.

[codecov-commenter]

Codecov Report

Merging #1718 (30a166e) into master (36ea904) will increase coverage by 0.04%.
The diff coverage is 60.86%.

@@            Coverage Diff             @@
##           master    #1718      +/-   ##
==========================================
+ Coverage   46.38%   46.42%   +0.04%     
==========================================
  Files         217      218       +1     
  Lines       12386    12414      +28     
==========================================
+ Hits         5745     5763      +18     
- Misses       6641     6651      +10     

Impacted Files	Coverage Δ
judge/admin/contest.py	44.73% <ø> (ø)
judge/views/ticket.py	39.44% <0.00%> (-0.74%)	⬇️
judge/views/two_factor.py	39.75% <ø> (+0.70%)	⬆️
judge/signals.py	64.19% <20.00%> (-2.91%)	⬇️
judge/admin/profile.py	62.22% <87.50%> (+1.98%)	⬆️
judge/migrations/0119_hide_problem_authors.py	100.00% <100.00%> (ø)
judge/models/contest.py	95.45% <100.00%> (+0.01%)	⬆️
judge/models/profile.py	90.90% <100.00%> (-0.71%)	⬇️
judge/forms.py	44.32% <0.00%> (-0.49%)	⬇️
judge/jinja2/markdown/__init__.py	67.18% <0.00%> (-0.26%)	⬇️
... and 2 more

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[Ninjaclasher]

Tested and works.

[kiritofeng]

LGTM