R1:7abdf3afe0a8

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…