The initializers including this were doing so at the top level, so every object
loaded after them had a `current_application_settings` method. However, if
someone had rack-attack enabled (which was loaded before these initializers), it
would try to load the API, and fail, because `Gitlab::CurrentSettings` didn't
have that method.
To fix this:
1. Don't include `Gitlab::CurrentSettings` at the top level. We do not need
`Object.new.current_application_settings` to work.
2. Make `Gitlab::CurrentSettings` explicitly `extend self`, as we already use it
like that in several places.
3. Change the initializers to use that new form.
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`
