PLT-2525: Render attachment fields similar to Slack
PLT-2525: Render attachment fields similar to Slack

* Render attachment fields similar to Slack

* Add /loadtest json url command

This allows us to easily create test posts with 
more props like Slack attachments

rickbatka/mattermost-webapp,rickbatka/mattermost-webapp,rickbatka/mattermost-webapp,rickbatka/mattermost-webapp