In v16.8.0, fix pages auth when namespace-in-path is enabled

What does this MR do?

In v16.8.0, fix pages auth when namespace-in-path is enabled

Related to: gitlab#439679 (closed)

Changelog: fixed

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines