User Story 10, User can Login

[memcmahon]

As a visitor
When I visit the login path
I see a field to enter my email address and password
When I submit valid information
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 I am logged in

DO THESE STORIES FIRST:

• User Story 7, User Registration
• User Story 2, Visitor Navigation

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 turingschool-projects#274

depends on turingschool-projects#265

depends on turingschool-projects#271