571 - Implement Stickybuttons

Play around with it at: http://stickybuttons.dokku.hypothes.is/docs/help

Meant to resolve: https://github.com/hypothesis/h/issues/571
When a Always-on or the Highlight tool is selected from the control bar, the buttons should remain visible to indicate they are ON. This will mean the user won't have to open the control bar again to turn them off.