Static files from content packs served from CONTENT_ROOT

[benjaoming]

Summary

• Reduces HTTP logging on dev server to something readable
• Fixes kalite manage contentpackchecker all --update so it only updates installed content packs
• Unpacks subtitle files *.vtt to CONTENT_ROOT/srt: Annotates with updated url and no longer needs to ignore any file or folder patterns during collectstatic

TODO

If not all TODOs are marked, this PR is considered WIP (work in progress)

• [x] Have tests been written for the new code? If you're fixing a bug, write a regression test (or have a really good reason for not writing one... and I mean really good!)
• [x] Has documentation been written/updated?
• [x] New dependencies (if any) added to requirements file
• [x] Update get_subtitle_file_path
• [x] Remove old code unpacking/updating /perseus/ke/exercises/*
• [x] In kalite.management.commands.setup, remove collectstatic_ignores
• [x] Update content annotation to discover from different dir and update url column
• [x] Update ka-lite-raspberry-pi Nginx Conf
• [x] Update dev server runserver
• [x] Update cherrypy staticdirs

Reviewer guidance

Let me know if you find anything unacceptable here. New static files handing will be described in release notes.

Issues addressed

5073

[codecov-io]

Current coverage is 51.82% (diff: 40.00%)

Merging #5386 into develop will increase coverage by 0.14%

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+ Misses         3632       3609    -23   
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[benjaoming]

Alright, seems it was just a matter of moving srt/ from STATIC_ROOT to CONTENT_ROOT in the unpacking process and then the annotation process.

[benjaoming]

Since the .srt files are now handled in CONTENT_ROOT, there's no need to update the folders served by the dev server, cherrypy or Nginx. Users having setup other mechanisms for serving these folders will also not be affected. This is great :)

[benjaoming]

Tested after reinstalling the English content pack that subtitles are still working