Implement the design changes from https://trello.com/c/DZEDxU4C/90-top-bar-ui-updates

 - Move the 'My Annotations' item up and change the label
   to the account username
 - Swap Feedback and Help
 - Make the 'Sign out' link more subtle

 * BEM-ify the dropdown menu link CSS for consistency
   and to avoid specificity conflicts