Show acct instead of username where applicable

This allows users to see the instance used by the poster in the
timeline, and I like the look of it better.

src/View/Common.elm

@@ -42,7 +42,7 @@ accountLink external account =
             [ href account.url
             , accountHref
             ]
-            [ text <| "@" ++ account.username ]
+            [ text <| "@" ++ account.acct ]
 
 
 accountAvatarLink : Bool -> Account -> Html Msg

src/View/Status.elm

@@ -218,7 +218,7 @@ statusView context ({ account, content, media_attachments, reblog, mentions } as
                     , div [ class "username" ]
                         [ a accountLinkAttributes
                             [ text (if account.display_name=="" then account.username else account.display_name)
-                            , span [ class "acct" ] [ text <| " @" ++ account.username ]
+                            , span [ class "acct" ] [ text <| " @" ++ account.acct ]
                             ]
                         ]
                     , Lazy.lazy2 statusContentView context status