Support setting Redis prefix using environmental variable REDIS_PREFIX

[christaikobo]

Redis does ACL using key and channel prefixes, if multiple apps and services were to use the same Redis instance as cache, the only way to enforce some kind of isolation and permission control between these apps and services is to set appropriate prefixes in them and control the permission in Redis.

This PR introduces a environmental variable REDIS_PREFIX, and adds corresponding prefixes for Django, Celery and Yamtrack's own session keys and channels.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.22%. Comparing base (28a9a93) to head (943dd6b).
⚠️ Report is 114 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/config/settings.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #834   +/-   ##
=======================================
  Coverage   83.22%   83.22%           
=======================================
  Files          69       69           
  Lines        6832     6838    +6     
=======================================
+ Hits         5686     5691    +5     
- Misses       1146     1147    +1     

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[FuzzyGrim]

Thanks for the PR!