Add a blanket "will login" event
Summary:
Ref T1536. Facebook currently does a check which should be on-login in registration hooks, and this is generally a reasonable hook to provide.
The "will login" event allows listeners to reject or modify a login, or just log it or whatever.
NOTE: This doesn't cover non-web logins right now -- notably Conduit. That's presumably fine.
(This can't land for a while, it depends on about 10 uncommitted revisions.)
Test Plan: Logged out and in again.
Reviewers: wez, btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T1536
Differential Revision: https://secure.phabricator.com/D6202