Target acquired.

Allows for Dares to be associated with "Target" users, or "Subjects." Identifying by email.
* If the email field is left blank, the Dare is open and up for grabs.
* If the email field is linked with an existing user, that user is now the "Target" of this Dare.
* If the email field is not linked with any user, a new user is created with the given email and a random password. He is then linked to this Dare.

Naturally, this means we need better user authentication.

1. Implement email verification.
2. Allow users to claim accounts created under their emails through new Dares.
3. Update user creation page to reflect this.

Reviewer: @nishadsingh1