Add a dropdown menu for Differential view options

Summary:
There are several open Differential tasks that are basically blocked on not
having reasonable places in the UI to put things. Replace the "View Standalone /
Raw" button with a "View Options" dropdown menu so we can shove things like
"Expand All", "Fold / Unfold File", and "View in Diffusion" in there.

This doesn't change any behavior, just puts the existing options in a menu.

Test Plan:

• Toggled menu open by clicking button.
• Clicked menu items.
• Toggled menu closed by clicking button.
• Toggled menu closed by clicking document.
• Toggled menu closed by opening another menu.
• Toggled menu closed by selecting an item.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Maniphest Tasks: T497, T309

Differential Revision: https://secure.phabricator.com/D1316