User Story 12, Users who are logged in already are redirected

[iandouglas]

As a registered user, merchant, or admin
When I visit the login path
If I am a regular user, I am redirected to my profile page
If I am a merchant user, I am redirected to my merchant dashboard page
If I am an admin user, I am redirected to the home page of the site
And I see a flash message that tells me I am already logged in

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