email: Reject tickets for unregistered users

This is a slight edit over the previous implementation of the same feature.
Previously, an "account" was required, which implies a user account with a
password. This implementation simply requires a user record. Importing of
users by organization domain is still supported -- even if the user does not
yet exist.
Accept subdomains in the organization domain box

Use .domain.tld to accept all subdomains of a parent
orm: Fix for the `contains` field filter
Fixup wording for unregistered email rejection config
Merge pull request #780 from greezybacon/issue/unregistered-email

Registered email includes users without an "account"

Reviewed-By: Peter Rotich <peter@osticket.com>