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]

Codecov Report

Attention: Patch coverage is 58.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 33.62%. Comparing base (0401c53) to head (025cf30).
Report is 22 commits behind head on master.

Files	Patch %	Lines
cms/server/contest/submission/workflow.py	0.00%	5 Missing ⚠️
cms/server/jinja2_toolbox.py	0.00%	3 Missing ⚠️
cms/server/admin/handlers/task.py	0.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (0401c53) and HEAD (025cf30). Click for more details.

HEAD has 4 uploads less than BASE

Flag	BASE (0401c53)	HEAD (025cf30)
unittests	3	1
functionaltests	2	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1209       +/-   ##
===========================================
- Coverage   63.88%   33.62%   -30.26%     
===========================================
  Files         233      234        +1     
  Lines       17151    17174       +23     
===========================================
- Hits        10957     5775     -5182     
- Misses       6194    11399     +5205     

Flag	Coverage Δ
functionaltests	?
unittests	33.62% <58.33%> (-10.42%)	⬇️

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