Summary:
- Adds a new ChoiceList type, and widgets to view and edit it.
- Store in SQLite as a JSON string
- Support conversions between ChoiceList and other types
Test Plan: Added browser tests, and a test for how these values are stored
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D2803
