join ohmlet via link in email for new account causes a 404

[jshslsky]

When the user clicks an ohmlet invite link in their email client, the app is supposed to intercept the link and perform some steps with the server.

If the user does not have an account, let the user create an account using the account creation screen or google. If they are creating an ohmage account, include the user_invitation_id in the POST if the user does not change the email address from what is in the invitation link. The user_invitation_id causes the email verification step to be skipped.

If the user is logged out and logs in or if the user is already logged in, the app should join the user to the ohmlet by parsing the join link and POSTing to /ohmlets/<ohmlet_id>/members?ohmlet_invitation_id=<the_invitation_id> with the following JSON object as the body

{ 
    "member_id" : "the users ohmage ID",
    "role" : "member" 
}

The use's authentication information should also be sent in the Authorization header.