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