Migrate ff_views values.

[jrobbins]

This is the other PR needed to resolve #2909.

In this PR:

• Mark enum values deprecated on the server side, but keep them available until after the script is run in prod.
• Implement a data migration script and register it in main.py
• For the old feature list page, the list of possible enums needs to be sorted because it is built up from a dictionary that is no longer in numeric order.