Modernize "favorite project policies" setting

Summary:
Ref T4103. Convert this into a proper internal setting and use transactions to mutate it.

Also remove some no-longer-used old non-modular settings constants.

Test Plan:

• Used policy dropdown, saw recently-used projects.
• Selected some new projects, saw them appear.
• Grepped for all removed constants.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16027