Task level languages

[SvizelPritula]

This pull request implements the feature suggested in #1155.

A checkbox is added to the task detail page in the AWS, which enables task level language restrictions. When enabled, this allows the organizers to disable some languages for the specified task.

[codecov[bot]]

Codecov Report

Merging #1209 (025cf30) into master (0401c53) will decrease coverage by 30.24%. The diff coverage is 58.33%.

@@             Coverage Diff             @@
##           master    #1209       +/-   ##
===========================================
- Coverage   63.86%   33.62%   -30.25%     
===========================================
  Files         233      234        +1     
  Lines       17151    17174       +23     
===========================================
- Hits        10954     5775     -5179     
- Misses       6197    11399     +5202     

Flag	Coverage Δ
functionaltests	?
unittests	33.62% <58.33%> (-10.41%)	:arrow_down:

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Impacted Files	Coverage Δ
cms/server/admin/handlers/task.py	0.00% <0.00%> (-44.33%)	:arrow_down:
cms/server/contest/submission/workflow.py	0.00% <0.00%> (-95.87%)	:arrow_down:
cms/server/jinja2_toolbox.py	1.43% <0.00%> (-82.39%)	:arrow_down:
cms/db/__init__.py	96.42% <100.00%> (ø)
cms/db/task.py	72.34% <100.00%> (-9.81%)	:arrow_down:
cmscontrib/updaters/update_45.py	100.00% <100.00%> (ø)
cms/server/admin/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
cms/grading/scoretypes/Sum.py	0.00% <0.00%> (-100.00%)	:arrow_down:
cms/server/contest/printing.py	0.00% <0.00%> (-100.00%)	:arrow_down:
cms/server/contest/tokening.py	0.00% <0.00%> (-100.00%)	:arrow_down:
... and 119 more

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