Update the themes to match the newmaster layout
    
    Changes the theme hooks so there is now only one variable
    to add items to the navbar at the top of the page (theme.masthead_class)
    
    Also change each of the themes to use only this variable, and change the
    default themes so they render the same as before the changes to
    master.html in
    https://pagure.io/pagure/c/bcd6a2b0a0deb29e0883c4fe61e37d4c27a75463?branch=master
    
    Related: #3614