Forget user when deleting an associated active session

[stevepolitodesign]

When deleting an active session I want to ensure the associated session is forgotten so that I can ensure my account it safe.

Acceptance Criteria

• Remove remember_token from users table.
• Add remember_token to active_sessions table.
• Refactor Authentication#forget.
• Refactor Authentication#remember.
• Refactor Authentication#current_user.