Lingering Chat Remark for Discord & Versioning

[MorrowShore]

This feature will add a remark in the chat box containing the Drawpile version and a link to the official DP server. (https://drawpile.net/discord/) This remark will act just like a normal chat message, so it'll be buried once the chatroom gets active. This is a very harmless addition that can be used to convey a variety of information, including the two mentioned here.

May get obliterated with the chat overhaul #1208, so another implementation will be adapted for it when it begins.