- Fixed bug 4916: users can have disabled themes.  Modified patch by Mathias.

   1. Remove the theme object.  There is no need to keep it around since meta
      information for a theme can be retrieved via list_themes().  All we really
      need is the theme name.

   2. Check if the user selected theme is enabled during theme initialization.
      This is the easiest place to put the check and doesn't mess with the user's
      settings.  Their database profile will still contain the disabled theme
      selection, but they will be rendering the default admin-selected theme
      until their chosen theme is once again activated.