R1:61a0c6d6e389

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…