Facebook email verification

This PR gets the client ready for our facebook email verification system that augments the giveaway.  It adds an action-link that will be used in the verification email that we send to users, and it introduces the 'invalid_fb_email_token' and 'fb_email_unverified' reward states.
It was already being handled, the error was just being cleared when the reward was toggled on.
Fixed in https://github.com/stellar/stellar-client/commit/f52e8482889fe90db8ce1531190a2cd6b236f09b.