Try to prevent autolinking of displaynames by email readers (#19169) (#19183)

Try to prevent autolinking of displaynames by email readers (#19169) (#19183)

Backport #19169

Unfortunately many email readers will (helpfully) detect url or url-like names and
automatically create links to them, even in HTML emails. This is not ideal when
usernames can have dots in them.

This PR tries to prevent this behaviour by sticking ZWJ characters between dots and
also set the meta tag to prevent format detection.

Not every email template has been changed in this way - just the activation emails but
it may be that we should be setting the above meta tag in all of our emails too.

Signed-off-by: Andrew Thornton <art27@cantab.net>