Add option to send confirm and migrate email to already-confirmed accounts

In these emails, we simply link directly to Special:MergeAccount instead of
Special:MergeAccount/sometoken. Without token, login is required, but it would
be anyway to get any account merged. The email still gives the option to
invalidate/unconfirm their email address if they choose to do so.

Bug: 54760
Change-Id: Ib674ef68e0c1b11a733379fe0178c253f06e4b91
Merge "Add option to send confirm and migrate email to already-confirmed accounts"