Update logic for forcing login on anonymous join attempt.

[simoneduca]

Style join button.

[shaunanoordin]

• Testing on localhost:3000+OSX+Chrome52
• As a non-logged in user, if I attempt to access a Classroom Join link (e.g. http://localhost:3000/students/join?id=...&token=...) I will be greeted with the Login prompt
• As a logged-in user, if I attempt to access a Classroom Join link, I will be greeted with the (newly styled) Join page with the Join button.
  • Clicking the Join button will direct me to the Map upon success (as expected), whether or not I have already joined that class.

Status: LGTM Actions: Merging