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.

* After signing up with Persona, a new Django User object and a new
  SocialAccount object are created, and populated with the correct data.

* After signing up with Persona, auth-related UI elements show Persona
  identification.