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.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1569

Differential Revision: https://secure.phabricator.com/D3105