Implement feedback models

[ozer550]

Summary

Implements feedback models to support Flagging, RecommendationsEvent and RecommendationdInteraction events.

Description of the change(s) you made

• Added decided hierarchy of models using abstract models.
• Unit tests for creation of models.

References

closes #4477

Contributor's Checklist

PR process:

• If this is an important user-facing change, PR or related issue the CHANGELOG label been added to this PR. Note: items with this label will be added to the CHANGELOG at a later time
• If this includes an internal dependency change, a link to the diff is provided
• The docs label has been added if this introduces a change that needs to be updated in the user docs?
• If any Python requirements have changed, the updated requirements.txt files also included in this PR
• Opportunities for using Google Analytics here are noted
• Migrations are safe for a large db

Testing:

• Code is clean and well-commented
• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Any new interactions have been added to the QA Sheet
• Critical and brittle code paths are covered by unit tests

[bjester]

Test performance can likely be increased, but otherwise looks good

[bjester]

Good clean work! Nice job