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/actions/global_actions.jsx

@@ -397,7 +397,7 @@ export function sendEphemeralPost(message, channelId) {
         user_id: '0',
         channel_id: channelId || ChannelStore.getCurrentId(),
         message,
-        type: Constants.POST_TYPE_EPHEMERAL,
+        type: Constants.PostTypes.EPHEMERAL,
         create_at: timestamp,
         update_at: timestamp,
         props: {}