User Story 9, Registration Email must be unique

[iandouglas]

As a visitor
When I visit the user registration page
If I fill out the registration form
But include an email address already in the system
Then I am returned to the registration page
My details are not saved and I am not logged in
The form is filled in with all previous data except the email field and password fields
I see a flash message telling me the email address is already in use

DO THESE STORIES FIRST:

• User Story 7, User Registration

Checklist:

• base tests are written
• base code is written to pass tests
• edge case ("sad path") tests written (if required)
• edge case code written (if required)
• code is reviewed
• branch is merged
• all tests still pass

child of #266

child of #274

depends on #265