Skip link for staff interfaces

Branch with separate commits for Angular and AngularJS screens. Improvements welcome, especially on AngularJS, where focus lands on the main content container in a way that's invisible.

https://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/sleary/2017034-staff-skipnav

To test:

1. If you don't have keyboard tabbing set up in your browser already, you'll need to do that first: https://wiki.evergreen-ils.org/doku.php?id=accessibility:testing#testing_keyboard_navigation
2. Open any AngularJS staff page
3. Focus on your address bar
4. Press the tab key until you get past the built-in browser buttons. The first thing you should see focused in the Evergreen screen itself is a "Skip to main content" link that is visible only when you're focused on it; when you press tab again, it should disappear.
5. If you press Enter while focused on the skip link, your focus should move past the navigation menu and onto the next element on the page, whatever that is.