User Story 8, User Registration Missing Details

[BrianZanti]

As a visitor
When I visit the user registration page
And I do not fill in this form completely,
I am returned to the registration page
And I see a flash message indicating that I am missing required fields

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