User Story 19, User Editing Profile Data must have unique Email address

[memcmahon]

As a registered user
When I attempt to edit my profile data
If I try to change my email address to one that belongs to another user
When I submit the form
Then I am returned to the profile edit page
And I see a flash message telling me that email address is already in use

DO THESE STORIES FIRST:

• User Story 17, User Profile Show Page
• User Story 18, User Can Edit their Profile Data

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 #253

child of turingschool-projects#274

depends on #252

depends on #251