In preparation for implementing the new top-bar
design, extract it into its own directive.

 * Refactor the 'persona' filter into a simple
   function that returns a {username, provider}
   object which IMO makes for more straightforward
   code.

   Several of the test cases which deal with inputs
   not in that form were removed.

   If account IDs are not guaranteed to be in
   the 'acct:<username>@<provider>' form then
   we should be explicit about what is accepted
   and what is returned.

T-90