Adds a slash command in slach for closing issues. See
https://docs.gitlab.com/ee/integration/slash_commands.html
for documentation on the wider feature set.
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`