Refactor platform/offered_by filtering and serialization for search

[mbertrand]

What are the relevant tickets?

Closes #237
Closes #113
Partially addresses #199

Description (What does it do?)

• Renames LearningResourcePlatform.platform to LearningResourcePlatform.code
• Refactors serialization and index schema for platform, offered_by
• Adjusts db api and search api to both use code property when filtering
• Adds/updates api schema documentation for the above fields.

How can this be tested?

Run docker compose up, some db migrations will run
Run ./manage.py recreate_index --all

If you haven't already, run the following mgmt commands:

./manage.py backpopulate_prolearn_data
./manage.py backpopulate_mitxonline_data

Try the following URL's they should all work and filter by the specified offeror/platform correctly. The total number of results should be the same, though the order will likely differ unless you add a sortby option.

http://localhost:8063/api/v1/learning_resources/?platform=mitxonline
http://localhost:8063/api/v1/learning_resources_search/?platform=mitxonline

http://localhost:8063/api/v1/learning_resources/?offered_by=mitpe
http://localhost:8063/api/v1/learning_resources_search/?offered_by=mitpe

Check the schema documentation, the codes and values for platform, offered_by, department, and resource type should be documented for both the db and opensearch endpoints.

http://localhost:8063/api/v1/schema/redoc/#tag/learning_resources
http://localhost:8063/api/v1/schema/redoc/#tag/learning_resources_search

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (60887cf) 76.74% compared to head (57e9242) 76.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #240      +/-   ##
==========================================
- Coverage   76.74%   76.70%   -0.04%     
==========================================
  Files         306      306              
  Lines       13523    13537      +14     
  Branches     2215     2219       +4     
==========================================
+ Hits        10378    10384       +6     
- Misses       2882     2887       +5     
- Partials      263      266       +3     

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[abeglova]

LGTM