R1:c40bc0c8bf75

Replace the primary scrollbar with a fake one to prepare for a persistent chat column

Summary:
Ref T7014. With a mouse plugged in, multi-panel UIs are pretty hideous on OSX. This is somewhat offputting for me in Conpherence, and really jumps out at me with the new column mocks in T7014.

Sites like Twitch and Facebook approach this by emulating the touchpad scrollbar to achieve a more aesthetic UI. Use a similar approach.

This:

- Replaces the main scrollbar with a prettier fake one.
- This prepares…