Invite user to a game

[Amar-Gill]

send an invite via email for another user to play a game with you:

• [ ] select a game from games index and click invite btn
• [ ] enter user email in the field
• [ ] P2 gets email with links to sign up or login and join game
• [ ] if new user, P2 sent to games index page where current challenge will be visible
• [ ] if existing user, P2 sent to game challenge accept page where P2 can select up to three habits to track