feat: Add context_key property to library collection/container keys

[bradenmacdonald]

Any key for a content item (e.g. an XBlock, a container, a collection) that's in a library or course should have the .context_key property to get the key of the learning context (course or library) that contains it.

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[codecov]

Codecov Report

Attention: Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 93.77%. Comparing base (15ae68c) to head (a5bb183).
Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
opaque_keys/edx/keys.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #378      +/-   ##
==========================================
- Coverage   93.84%   93.77%   -0.08%     
==========================================
  Files          31       31              
  Lines        3007     3021      +14     
  Branches      192      192              
==========================================
+ Hits         2822     2833      +11     
- Misses        159      161       +2     
- Partials       26       27       +1     

Flag	Coverage Δ
unittests	93.77% <96.00%> (-0.08%)	⬇️

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[bradenmacdonald]

Closing in favor of #379