Allow setting rejection message for subscription request
This is similar to #17 (closed), but for rejection of subscription request to moderated lists.
As of now, when a user subscribes to a list which is "confirm, then moderate", the email sent to list-owner is "Your authroization is required for a mailing list subscription approval" + the email and list name.
When we go to postorius, under the "Subscription requests" -> "Pending approval" we do see the pending email.
But when we hit reject we don't get to enter a reason why we are rejecting. By default the user gets a "[No reason given]" reason.
Looking at the code base of core I am thinking that this message could probably be changed by editing the template list:user:notice:refuse - but if that is true then the template will be used in two similar but different places.