Customize the account age blocked message

[paolojpa]

Add the option to modify the message when you set up account age as a requirement.

The actual message is:

System Message: Recently Joined. User can try again 23 hours, 59 minutes and 44 seconds.

[Taaku18]

This repo is dedicated to the Modmail Wiki / documentation. Please direct this type of suggestions to the Modmail repo.

However, I'll transfer this issue this time.

[Taaku18]

This config suggestion isn't interfaced with the user and serves minimal purpose. Will not be added.