Bug 1059828: Begin adding tests for allauth workflows.

This consolidates and/or obsoletes some of our pre-existing tests with
new, more granular tests of the Persona workflow.

Tests added/consolidated here verify that:

* Persona authentication failure is handled gracefully.

* Persona authentication failure produces the correct error message.

* Persona authentication during signup properly leads into profile
  creation.

* Persona-based profile creation shows the correct Persona-relevant
  messages.

* Persona sign-in with an existing account redirects correctly (to
  home page or to page the user signed in from).

* After signing in with Persona, auth-related UI elements show Persona
  identification.

* Pages displayed to non-authenticated users come pre-loaded with the
  stub Persona form and relevant data attributes.
Just a heads-up: I added a `verify_strings_not_in_response` test helper for this in https://github.com/groovecoder/kuma/commit/b40ea3ac61640408aa3c4d59701a57b421cfafb1#diff-2 ... if it lands before this we could rebase and use it here.