Slack format for Slash command messages (#4999)

* Slash commands accept Slack format

Until this commit the slash commands only accepted 'text' properties.
For better styling however, Slack formatting support was added.

However, ephemeral messages are not supported, and only text will be
displayed.

* Allow emphemeral Slack messages

webapp/components/post_view/components/post_body_additional_content.jsx

@@ -136,7 +136,7 @@ export default class PostBodyAdditionalContent extends React.Component {
     }
 
     generateStaticEmbed() {
-        if (this.props.post.type === Constants.POST_TYPE_ATTACHMENT) {
+        if (this.props.post.props && this.props.post.props.attachments) {
             return this.getSlackAttachment();
         }