MM-11272 Add OpenGraph and image dimension metadata to posts (#9313)

* Move OpenGraph code into its own file

* Move OpenGraph image proxying to app layer

* Move test file code out of api4 package

* MM-11272 Add OpenGraph and image dimension metadata to posts

app/app.go

@@ -262,6 +262,8 @@ func New(options ...Option) (outApp *App, outErr error) {
 		handlers: make(map[string]webSocketHandler),
 	}
 
+	app.InitPostMetadata()
+
 	return app, nil
 }