Revise function invite_by_email_invite_presave

This issue originally was created for the following reason:

This has bugged me for quite a while, but I thought maybe there was some legitimate use for this. But over time I realized it's just a bug and needs to be removed...

invite_by_email's invite_presave overrides the invitee if a user exists with the email address of the invitation. In both cases of when this code actually makes a difference, the behavior is not desirable:

1)
- User A sends invite to email address associated with EXISTING USER B:
X New invitation is created with invitee already set to user B

2)
- User A sends invite to nonexistinguserb@example.com
- nonexistinguserb@example.com joins as userb without using the invite
- nonexistinguserc joins as userc using the invite
X invitee is set to userb instead of userc

The thing I *think* was intended was to catch cases where a user joins without using the invitation, and to match the user that way. But it's not achieving that goal anyway, and that would be best done in an invite_by_email_user_insert.

When I was reviewing the original patch I became frustrated because the code was more complex and difficult to understand than it needed to be.

Patch #6 fixes the issue and simplifies the code.