Warn when sending PMs to users who haven't been seen in a long time

We would like to add add small warning on the editor, in the place that warns users

You’re messaging someone who hasn’t been seen here in a very long time – {x=“two years”}. They may not receive your message. You may wish to seek out other methods of contacting this person.

Users absent from a community are very unlikely to respond, this helps protect them from unwanted emails and protect the senders from disappointment.