Support invites in the registration and login flow

Summary:
Ref T7152. This substantially completes the upstream login flow. Basically, we just cookie you and push you through normal registration, with slight changes:

• All providers allow registration if you have an invite.
• Most providers get minor text changes to say "Register" instead of "Login" or "Login or Register".
• The Username/Password provider changes to just a "choose a username" form.
• We show the user that they're accepting an invite, and who invited them.

Then on actual registration:

• Accepting an invite auto-verifies the address.
• Accepting an invite auto-approves the account.
• Your email is set to the invite email and locked.
• Invites get to reassign nonprimary, unverified addresses from other accounts.

But 98% of the code is the same.

Test Plan:

• Accepted an invite.
• Verified a new address on an existing account via invite.
• Followed a bad invite link.
• Tried to accept a verified invite.
• Reassigned an email by accepting an unverified, nonprimary invite on a new account.
• Verified that reassigns appear in the activity log.

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Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7152

Differential Revision: https://secure.phabricator.com/D11737