R1:57a072263639

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