Summary: allow GitLab users to publish private GitLab pages

# Background

In our GitLab instance, all GitLab pages are public, that is sites published by GitLab CI outside of the static-component system have no access control whatsoever.

GitLab pages does support enabling authentication to hide pages under GitLab authentication. This was not enabled in our instance.

# Proposal

Enable the GitLab access control mechanisms under the `read_api` scope.

Note that this might make your GitLab pages inaccessible if your project was configured to hide them. If that's not something you want, head to `Settings` -> `General` -> `Visibility` -> `Pages` and make them public.

# Deadline

This was implemented on 2023-02-16, and this proposal was written to retroactively inform people of the change.

# Status

This proposal is currently in the `standard` state.

# References

Discussion in [tpo/tpa/team#41079][].

[tpo/tpa/team#41079]: https://gitlab.torproject.org/tpo/tpa/team/-/issues/41079