PLT-2555/PLT-5009/PLT-5225 Changed system messages to be rendered by the client (#5209)

* Moved rendering of (message deleted) into PostMessageView

* Added additional post types to constants on client

* Changed system messages to be rendered in the client's language

* Updated new system messages to have relevant usernames highlighted and have markdown rendered in header change messages

webapp/utils/post_utils.jsx

@@ -73,4 +73,4 @@ export function canEditPost(post, editDisableAction) {
         }
     }
     return canEdit;
-}
+}