add model.NewX funcs for builtin types (#7692)

* add model.NewX funcs for builtin types

* whoops, forgot to add the new file

api/post_test.go

@@ -307,8 +307,7 @@ func testCreatePostWithOutgoingHook(
 		}
 
 		resp := &model.OutgoingWebhookResponse{}
-		resp.Text = new(string)
-		*resp.Text = "some test text"
+		resp.Text = model.NewString("some test text")
 		resp.Username = "testusername"
 		resp.IconURL = "http://www.mattermost.org/wp-content/uploads/2016/04/icon.png"
 		resp.Props = map[string]interface{}{"someprop": "somevalue"}