Add the code to registration to handle invitation codes.
Some edge cases on invitations: is the user already subbed to this
person? Tell them. Is the person already on the system? Sub the user
to them, then, and tell the user.
Add some code to User to auto-sub invitees whenever the email address
changes. Call it from a new registration with an invite code, and also
from confirmaddress.
Some whitespace cleanup in the files touched.
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