Make transcripts indexable for LMS search

[wowkalucky]

Added

• index_dictionary xblock method which makes xblock indexable for search;
• simple vtt_to_txt xblock method to extract transcripts content for indexing;

[codecov]

Codecov Report

Merging #294 into dev will decrease coverage by <.01%.
The diff coverage is 5.71%.

@@            Coverage Diff             @@
##              dev     #294      +/-   ##
==========================================
- Coverage   86.14%   86.14%   -0.01%     
==========================================
  Files          42       42              
  Lines        2758     2793      +35     
  Branches       26       26              
==========================================
+ Hits         2376     2406      +30     
- Misses        379      384       +5     
  Partials        3        3

Flag	Coverage Δ
#acceptance	89.03% <5.71%> (-1.18%)	⬇️
#unit	84.46% <5.71%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
video_xblock/mixins.py	86.42% <14.28%> (-2.36%)	⬇️
video_xblock/video_xblock.py	63.96% <3.57%> (-6.04%)	⬇️
...static/js/studio-edit/transcripts-manual-upload.js	36.36% <0%> (+9.09%)	⬆️
.../spec/studio-edit/transcript-manual-upload-spec.js	100% <0%> (+9.75%)	⬆️
video_xblock/static/js/studio-edit/utils.js	92.85% <0%> (+21.42%)	⬆️
...eo_xblock/static/js/spec/studio-edit/utils-spec.js	94.11% <0%> (+35.29%)	⬆️

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