Consolidate FxA and Persona post-login handlers (bug 1055555)

https://bugzilla.mozilla.org/show_bug.cgi?id=1055555

Use the same login handler for Persona and FxA. This fixes the views not updating bug because the FxA handler wasn't calling `require('views').reload()`.
`logInFailed` has a default message, so that will be used.