Add optional opaque "rpid" paramter to verification emails.

Strawman for how the auth server can assist with #309

This allows requests that generate a verification email to include an optional "rpid" parameter, for use as a "Relying Party Identifier".  It's opaque to the auth server and is just included verbatim as a query parameter in the verification URL.

The idea is that the content-server can assign rpids via the magic of not-defined-in-this-protocol, submit the rpid during account creation, and use it when displaying the verification page in order to customize the messaging appropriately.

@ckarlof @dannycoates thoughts?  Total strawman so so feel free to brutally destroy it ;-)