User Story 11, User cannot log in with bad credentials

[iandouglas]

As a visitor
When I visit the login page ("/login")
And I submit valid information
Then I am redirected to the login page
And I see a flash message that tells me that my credentials were incorrect
I am NOT told whether it was my email or password that was incorrect

DO THESE STORIES FIRST:

• User Story 10, User can Login

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 #262

child of #274

depends on #261