Create abstract form API

[avishkar58]

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PLACEHOLDER LONGER-TERM CLEANUP

The application form, review form and registration form all have very similar data models (form, sections, questions) and have similar logic and some shared components on the front-end. Create a more abstract version of a form with sections and questions that covers the use cases of all three. This will reduce code duplication and improve reliability through testing. It will be even more important when we introduce complex form-builder UI logic.

This supersedes #603