Add mypy annotations

[Ninjaclasher]

Supercedes #1495. Refs #1493.

[codecov-commenter]

Codecov Report

Patch coverage: 93.35% and project coverage change: +1.12% :tada:

Comparison is base (79be4af) 46.84% compared to head (ba67723) 47.96%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2283      +/-   ##
==========================================
+ Coverage   46.84%   47.96%   +1.12%     
==========================================
  Files         249      249              
  Lines       13158    13453     +295     
==========================================
+ Hits         6164     6453     +289     
- Misses       6994     7000       +6     

Files Changed	Coverage Δ
dmoj/celery.py	81.25% <0.00%> (+1.25%)	:arrow_up:
judge/bridge/base_handler.py	0.00% <0.00%> (ø)
judge/event_poster_ws.py	0.00% <0.00%> (ø)
judge/management/commands/generate_sitemap.py	0.00% <0.00%> (ø)
judge/management/commands/runmoss.py	0.00% <0.00%> (ø)
judge/utils/pdfoid.py	45.00% <ø> (ø)
judge/utils/problem_data.py	15.02% <0.00%> (+0.40%)	:arrow_up:
judge/widgets/select2.py	56.04% <ø> (+0.98%)	:arrow_up:
judge/views/tasks.py	42.55% <50.00%> (+1.24%)	:arrow_up:
judge/views/two_factor.py	40.22% <50.00%> (+0.34%)	:arrow_up:
... and 28 more

... and 43 files with indirect coverage changes

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