Memory: Start migrating searchbar JSON out of Educator model

[kevinrobinson]

Who is this PR for?

everyone

What problem does this PR fix?

There's a lot of data on the Educator model precomputed for the search bar that is otherwise never used when querying the Educator model. This means we're allocating lots of text we never need.

What does this PR do?

Starts the migration, adding EducatorSearchbar to hold this and updating the controller code to go through EducatorSearchbar. Deployment sequence:

first:

• set USE_SEARCHBAR_JSON_ON_EDUCATOR_MODEL=true
• deploy!
• verify querying new endpoint
• run rake task to write to new field
• cut over to query from new field (remove USE_SEARCHBAR_JSON_ON_EDUCATOR_MODEL)

next PR (#2460):

• remove old endpoint
• remove old read and feature switch
• remove column
• remove old as_json optimization

Checklists

Which features or pages does this PR touch?

• Core, search bar

Does this PR use tests to help verify we can deploy these changes quickly and confidently?

• Included specs for changes
• Improved specs for existing code in need of better test coverage
• Manual testing made more sense here

[kevinrobinson]

selfie