Adjustments to allow for Redis+TLS

[Ardiea]

Tweaking the application settings to allow celery and redbeat to use TLS when connecting to Redis/Elasticache.

Pre-Flight checklist

• [ ] Screenshots and design review for any changes that affect layout or styling
  • [ ] Desktop screenshots
  • [ ] Mobile width screenshots
• [ ] Migrations
  • [ ] Migration is backwards-compatible with current production code
• [X] Testing
  • [X] Code is tested
  • [X] Changes have been manually tested
• [ ] Settings
  • [ ] New settings are documented and present in app.json
  • [ ] New settings have reasonable development defaults, if applicable

What are the relevant tickets?

https://github.com/mitodl/ol-infrastructure/issues/1099

What's this PR do?

Documents an explicit distinction between REDIS_URL and CELERY_BROKER_URL and when they can be same (no TLS) and when the must diverge (using TLS) as well as the nature of that divergence.

Additionally updates celery-redbeat from a really old version to just an old version (but the newest available...)

How should this be manually tested?

... Might be tricky without REDIS+TLS. It would be good to verify that non-TLS setups continue to work by setting REDIS_URL.

TLS is straight forward and the default in AWS and it comes with a verifiable certificate. I can do a screenshare on a new QA environment that has REDIS+TLS if you want to see it in action.

Where should the reviewer start?

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Screenshots (if appropriate)

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[codecov-commenter]

Codecov Report

Merging #1004 (ce28460) into master (1be76e2) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1004   +/-   ##
=======================================
  Coverage   96.03%   96.03%           
=======================================
  Files         182      182           
  Lines        7345     7345           
  Branches      251      251           
=======================================
  Hits         7054     7054           
  Misses        257      257           
  Partials       34       34           

Impacted Files	Coverage Δ
odl_video/settings.py	87.77% <100.00%> (ø)

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