Allow sidenav to have custom classes on items
Summary:
I need this to set "disabled" on some menu items that are policy-restricted.
NOTE: This is getting gross and I promise to clean it up with the new side nav stuff.
Test Plan: Added "Disabled" to some items, they became disabeld.
Reviewers: vrana, btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T603
Differential Revision: https://secure.phabricator.com/D3203