Implement authenticated encryption in community auth

Implement authenticated encryption in community auth

This creates a community auth version 3 (previous one being 2, and 1 is
long gone) trhat uses AES_SIV as the encryption method instead of
regular AES_CBC, and validates the digests on all accounts.

As this gets deployed on servers incrementall, the version has to be
specified in the database record for the site. We could have the site
indicate this itself, but doing it this way seems safer as it will then
just break for any app that accidentally reverts the plugin.

Reviewed by Jacob Champion