Add login page

[acrantel]

Adds the login page.

Testing:

• Try logging in with invalid username / pwd. Error message should appear
• Try logging in with correct username / pwd. Login should succeed and should redirect to the home page of the most recently accessed episode.
• Try going to the /login page while already logged in. It should redirect you to the home page of the most recently accessed episode.

Note: testing is blocked on https://github.com/battlecode/galaxy/pull/667.

[n8kim1]

test-ready now?