Begin rebuilding dropdown menus on ActionList

Summary:
Dropdown menus are entirely dynamic right now and use custom CSS. Begin rebuilding them to use ActionList CSS.

This introduces PHUIX components which are basically JS copy/pastes of the PHP PHUI components, just implemented in JS.

We have two other dropdowns: policy controls and one in Conpherence. I'll convert those, then implement D8966.

Test Plan: {F150418}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8973