feat: context key property for new container locator keys

[navinkarkera]

1. All keys for content items inside a learning context should have the .context_key property to get the parent learning context, but some of the new library keys were missing this.

2. Adds a new Django field type that can store LibraryItemKey (Collection or Container keys). (TODO: this should probably be able to hold library usage keys too, since those are also library items)

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[codecov]

Codecov Report

Attention: Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 93.64%. Comparing base (15ae68c) to head (168abc6).
Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
opaque_keys/edx/locator.py	66.66%	2 Missing ⚠️
opaque_keys/edx/keys.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #375      +/-   ##
==========================================
- Coverage   93.84%   93.64%   -0.21%     
==========================================
  Files          31       31              
  Lines        3007     3022      +15     
  Branches      192      192              
==========================================
+ Hits         2822     2830       +8     
- Misses        159      165       +6     
- Partials       26       27       +1     

Flag	Coverage Δ
unittests	93.64% <81.25%> (-0.21%)	⬇️

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[kdmccormick]

As I understand it from the discussion on openedx/frontend-app-authoring#1711 (comment), we will not need to emulate container keys as usage keys, so I think we can close this PR in favor of openedx/openedx-platform#36553, right?

[navinkarkera]

@kdmccormick Yes, we don't need most of it but adding context_key property to container keys is useful.

[navinkarkera]

Closing in favor of #379