Slack slash commands using Slack app integration for GitLab on premise

Description

• Slack app for on premise to do slash commands. The on premise version of https://gitlab.com/gitlab-org/gitlab-ce/issues/32167.
• Analogus on-prem Slack apps for JIRA are
  • https://slack.com/apps/A0F7YS3MZ-jira-server-alerts
  • http://jirioslackapp.com