Added Mattermost

Author: menewol

objects/instant-message/definition.json

@@ -22,7 +22,8 @@
         "Discord",
         "Mumble",
         "Jabber",
-        "Twitter"
+        "Twitter",
+        "Mattermost"
       ],
       "ui-priority": 1
     },
@@ -118,4 +119,4 @@
   ],
   "uuid": "5fa51a24-f40f-4696-a77e-d31e26bab5fc",
   "version": 3
-}
+}