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`
This commit was about 6 commits before squashing, with the main goal to
make all tests green. Now, after pushing this commit we'll see what the
CI has to say about that.