Retrieves the config value named CONFIG, parses it as a Text::Template,
extracts "to" and "subject" headers, and returns a hash that can be passed
-to L<FS::Misc::send_email>.
+to L<FS::Misc/send_email>.
PARAMS is a hashref to be passed to C<fill_in>. It must contain
'agentnum' to look up the per-agent config.