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]
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
