Allows a button type for menus, used in Conpherence

Summary: This adds a new menu item, TYPEBUTTON, for use in Conpherence and maybe others over time. Note I need to add icon support, but I'll make them later tonight. I also changed the front facing names to 'Conversations' which is way more natural. Basically, Pholio has Mocks, Differential has Diffs, Conpherence has Conversations.

Test Plan: Tested Conpherence on mobile and desktop.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2430

Differential Revision: https://secure.phabricator.com/D4691