Loading state Slack response after slash command

• As confirmed with Slack, it is a good practice to send back a loading state Slack response after a slash command has been executed.
• For example, after you create an issue with a slash command, a loading state message could be Command received. Creating issue.
• And then when the issue is created, then GitLab sends back the JSON payload to display the full message inside Slack.
• It should be possible to replace the loading message with the issue display message once it arrives: https://api.slack.com/methods/chat.update or https://api.slack.com/methods/chat.delete. This is the design practice to follow per Slack.