R1:3c7944d297b2

Switch to new menubar

Summary:
- Looks better (can probably still use some tweaks), especially search.
- Moves logout from weird footer location to main menu.
- Reactive: on tablets and phones, the menu adjusts to remain useful.
- Fixed position on desktops for future side nav changes.
- Adds an icon header thing that's currently hard-coded but will be application-driven soon.

Test Plan: Used menu on desktop, tablet, phone, logged in / logged out, toggled darkconsole. Will add some screenshots.