Fix task submission language option re-enable when reset

[edisonhello]

Language list will be disable when a file is selected, but will not re-enable when reset button is pressed.
This commit add a event listener on the button which calls CMS.CWSUtils.filter_languages to reset language list.

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This change is 

[codecov]

Codecov Report

Merging #1158 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1158      +/-   ##
==========================================
- Coverage   61.99%   61.91%   -0.08%     
==========================================
  Files         230      230              
  Lines       16604    16604              
==========================================
- Hits        10294    10281      -13     
- Misses       6310     6323      +13     

Flag	Coverage Δ
#functionaltests	45.63% <ø> (+0.03%)	⬆️
#unittests	43.29% <ø> (ø)

Impacted Files	Coverage Δ
cms/server/admin/handlers/contestuser.py	23.72% <0.00%> (-7.63%)	⬇️
cms/io/rpc.py	90.10% <0.00%> (-1.03%)	⬇️
cms/service/esoperations.py	78.01% <0.00%> (-0.71%)	⬇️
cms/server/admin/handlers/base.py	67.11% <0.00%> (-0.68%)	⬇️
cms/io/service.py	68.90% <0.00%> (-0.61%)	⬇️
cms/service/EvaluationService.py	67.02% <0.00%> (-0.54%)	⬇️
cms/grading/Job.py	88.62% <0.00%> (-0.48%)	⬇️
cms/db/base.py	84.04% <0.00%> (ø)
cms/db/filecacher.py	77.37% <0.00%> (+0.32%)	⬆️
cms/grading/Sandbox.py	68.23% <0.00%> (+0.36%)	⬆️
... and 2 more

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[andreyv]

Thanks, I can reproduce the issue.

I think there may be an even easier fix. Could you please check if this works:

1. Use the onreset event attribute (on <form>) instead of onclick.
2. Just .find() the <option> elements directly and call .removeAttr() on all of them at once.