R1:893243b7897b

Approximately rough in persistent chat column very roughly

Summary:
Ref T7014. This is very rough and not hooked up to anything, but gets a couple of the layout pieces in place so we can (a) see that it looks like it'll kinda work; (b) look for problematic interactions and (c) you can fix my mangling of your design.

NOTE: Press "\" to toggle the column.

Test Plan:
Feels pretty good to me?

{F275722}

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7014…