Add sync verification class to centralise knowledge of sync verification
Start moving re-arranging responsibilities for mapping

The Memberful_User_Map object should be solely concerned with mapping
memberful members onto WP users, and shouldn't be redirecting users
around.

The Authenticator should be responsible for redirecting and setting
cookies etc.
[mapping] Clarify that wp returns false when a user doesn't exist
[mapping] Fix default value of $user_member_is_mapped_to to match WP

get_user_by returns false when it can't find a user, not NULL
[mapping] Fix some references to typos and bugs
Fix syntax typo
Fix some bugs in modified user mapper
Fix variable typos when setting up the sync verification nonce
Require user to use login form to verify the Memberful sync link

This prevents an attacker from forcing the user to start the OAuth flow,
then "completing" the verification by using some other kind of SSO
service.
Ensure that the member is linked to the correct user after sync link

The checks at the top of the method verify that the user has authorized
the link. Follow-up commit will double check that the user who verified the
link is the one who is being mapped to.
Be explicit that user verifying link is the one being linked

In theory this isn't necessary as the the linking information is
retrieved from the meta of the user who signed in, but it doesn't hurt
to be a little explicit.
Ensure verified user's id is cast as int
Get rid of leftover mappins that point to the 0th wp user
Remove 'caller' from custom error logging

Too cumbersome, just use debug_backtrace()
Fix usage of posting data to Memberful API
Automatically include the backtrace in errors
Add context information to some errors during sign in and sync