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/utils.jsx

@@ -1170,7 +1170,7 @@ export function clearFileInput(elm) {
 }
 
 export function isPostEphemeral(post) {
-    return post.type === Constants.POST_TYPE_EPHEMERAL || post.state === Constants.POST_DELETED;
+    return post.type === Constants.PostTypes.EPHEMERAL || post.state === Constants.POST_DELETED;
 }
 
 export function getRootId(post) {