Add offline support

[adityacp]

• Add a new field in lesson model to add relative offline video path
• Add a Vue app to provide an offline interface. Vue app provided from https://github.com/CruiseDevice/offline_yaksh
• Create a zip file to download
• Requires API from the PR Update Yaksh API #674

[codecov-io]

Codecov Report

Merging #666 into master will decrease coverage by 0.95%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #666      +/-   ##
==========================================
- Coverage   81.50%   80.54%   -0.96%     
==========================================
  Files          36       36              
  Lines        5158     5110      -48     
  Branches      837      782      -55     
==========================================
- Hits         4204     4116      -88     
- Misses        641      697      +56     
+ Partials      313      297      -16     

Impacted Files	Coverage Δ
yaksh/forms.py	83.95% <100.00%> (-0.14%)	⬇️
yaksh/models.py	89.61% <100.00%> (-0.12%)	⬇️
yaksh/views.py	71.60% <100.00%> (+0.20%)	⬆️
yaksh/r_code_evaluator.py	0.00% <0.00%> (-79.42%)	⬇️
yaksh/middleware/user_time_zone.py	90.00% <0.00%> (-2.86%)	⬇️
yaksh/templatetags/custom_filters.py	90.16% <0.00%> (-1.64%)	⬇️
grades/views.py	93.93% <0.00%> (-0.35%)	⬇️
grades/models.py	94.73% <0.00%> (ø)
... and 5 more

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[CruiseDevice]

I think the offline app in this PR does not have support for int, str, float type questions. I have already created a PR for this in my own repo, but not merged.