This improves the output produced when running an unknown command,
running the "help" command, and when trying to run a command you are not
allowed to run. The new help output includes links to the project of the
chatops integration, and a link to the chatops documentation.
Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/27070
Deprecate "chat commands" in favor of "slash commands"
We looked for things like:
- `slash commmand`
- `slash_command`
- `slash-command`
- `SlashCommand`