Harden download robustness

[benjaoming]

Summary

• Fixes blank video downloads when server is down (part of Video download robustness improvements #5538)
• Raises exceptions when non-200 download responses
• Resumes video / content pack download in case of connection disruptions during download (not in case the download is stopped/started)
• Adds tests for various failures
• Adds tests for downloading + deleting content packs
• Deletes content pack database when deleting a content pack.
• Fixed missing update from database when a video download gets canceled while being processed
• Improves logging on many fronts
• Feedback when downloading content packs retrievecontentpack download gives no feedback #5356

TODO

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

• Has documentation been written/updated?
• Have you written release notes for the upcoming release?
• Allow to fail gracefully if thumbnails are absent

Reviewer guidance

...

Issues addressed

#5538
#5356

[codecov]

Codecov Report

Merging #5545 into 0.17.x will increase coverage by 1.89%.
The diff coverage is 73.14%.

@@            Coverage Diff             @@
##           0.17.x    #5545      +/-   ##
==========================================
+ Coverage   64.67%   66.56%   +1.89%     
==========================================
  Files         117      117              
  Lines        6612     6639      +27     
==========================================
+ Hits         4276     4419     +143     
+ Misses       2336     2220     -116

Impacted Files	Coverage Δ
kalite/updates/settings.py	100% <ø> (ø)	⬆️
kalite/topic_tools/content_models.py	77.8% <0%> (-0.29%)	⬇️
kalite/updates/models.py	84.82% <100%> (+0.13%)	⬆️
kalite/updates/management/utils.py	78.44% <100%> (+5.88%)	⬆️
...alite/updates/management/commands/videodownload.py	74.56% <62.5%> (+15.73%)	⬆️
kalite/updates/videos.py	88.73% <73.33%> (+2.79%)	⬆️
...ributed/management/commands/retrievecontentpack.py	89.16% <76.92%> (+40.98%)	⬆️
kalite/i18n/base.py	76.17% <81.25%> (+20.31%)	⬆️
... and 8 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 86e1ea6...e941add. Read the comment docs.

[benjaoming]

Added download progress for contentpack retrieval.

[benjaoming]

Download progress callback also added in UI

[benjaoming]

Repositioned the progress bars for content pack downloads so it doesn't look like something is broken.

[benjaoming]

A small related TODO of this PR would be to add automated testing for retrieving and deleting a content pack.

[benjaoming]

Merging this for further tests in a pre-release!