Slack slash commands

Description

Developers want to be able to issue commands to GitLab via a chat interface (aka ChatOps). Let's provide an MVP of a GitLab integration to Slack.

As a developer and product manager, slash commands integration (from #19838 (moved)) works better than bot integration. The slash commands feel more natural with Slack. There's no benefit by having a "bot" that users can talk with, @ mention, etc. The app backend could exist in the main Rails app of GitLab without requiring additional containers/processes/configuration.

Proposal

• Add a Slack webhook backend to GitLab CE
• Add instructions to configure slash commands in Slack
• Use this link in the setup text for “Add a slash command”: https://my.slack.com/services/new/slash-commands
• Add UI to enable webhook for GitLab project
• Support various read-only slash commands (e.g. /issue). Potential (incomplete) vocabulary:
  • /issue [ISSUE]
  • /merge-request
  • /pipeline
  • /snippets

Design

🔍 Check design specs (for spacing, sizes, colors and text copying)

MR review

Mention or assign @pedroms to the MR for UX review before ~Frontend review begins.

Links / references

• Reduced scope from #20799 (closed)