Add the ability to add/remove a chat background (Closes #1)

langs/en-ca.json

@@ -4,6 +4,10 @@
 			"startSidebarExpanded": {
 				"name": "Start Sidebar Expanded",
 				"hint": "(v13+) Starts the right-hand sidebar expanded when logging in."
+			},
+			"chatSidebarBackground": {
+				"name": "Chat Background",
+				"hint": "(v13+) Adds a background to the chat tab of the right-hand sidebar."
 			}
 		}
 	}