Show an account provider picker on the server confirmation screen when required. (#4137)

* Add a picker mode to the ServerConfirmationScreen.

* Hook up the account provider picker in authentication the flow.

Simplify the authentication flow coordinator, removing the restricted state.

* UI/Snapshot tests.

Package.resolved

@@ -22,8 +22,8 @@
       "kind" : "remoteSourceControl",
       "location" : "https://github.com/jpsim/Yams",
       "state" : {
-        "revision" : "3d6871d5b4a5cd519adf233fbb576e0a2af71c17",
-        "version" : "5.4.0"
+        "revision" : "9281f8c99aff4f4a55dce22ae29b1181c935caa5",
+        "version" : "6.0.0"
       }
     }
   ],