R1:462299388566

Give Phriction a real dropdown

Summary:
Fixes T8150. Small generalization:

- Timeline has a dropdown with no special logic; make that generic.
- Use it in Phriction.

Test Plan:
- Used Phriction dropdown on desktop and mobile.
- Used timeline dropdown.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8150

Differential Revision: https://secure.phabricator.com/D12931