This reverts merge request !20679

Enable frozen string in:

* app/presenters
* app/policies

Partially addresses #47424.

The `access_git` and `access_api` were currently never checked for
anonymous users. And they would also be allowed access:

  An anonymous user can clone and pull from a public repo

  An anonymous user can request public information from the API

So the policy didn't actually reflect what we were enforcing.

When terms are enforced, but the user has not accepted the terms
access to the API & git is rejected with a message directing the user
to the web app to accept the terms.

- Fix duplicate `prevent` declaration
- Add spec for `GlobalPolicy`

- Use `GlobalPolicy` to authorize the users that a non-authenticated user can
  fetch from `/api/v4/users`. We allow access if the `Gitlab::VisibilityLevel::PUBLIC`
  visibility level is not restricted.

- Further, as before, `/api/v4/users` is only accessible to unauthenticated users if
  the `username` parameter is passed.

- Turn off `authenticate!` for the `/api/v4/users` endpoint by matching on the actual
  route + method, rather than the description.

- Change the type of `current_user` check in `UsersFinder` to be more
  compatible with EE.

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`