We validate the *format* of email addresses in many places with a match against
a regex, often with very slightly different syntax.
Adding a separate EmailAddressValidator simplifies the code in a few spots and
feels cleaner.
Deprecated the old location in case someone is using it in a plugin.
No functionality change is in this commit.
Note: the regex used at the moment does not support using address literals, e.g.:
* localpart@[192.168.0.1]
* localpart@[2001:db8::1]
Michael Brown