Enable Gitlab-Pages for subgroups

What does this MR do?

This MR enables Gitlab-pages controllers for projects inside a subgroup.

In order to work the gilab-pages counterpart MR gitlab-pages!123 (merged) is needed

What are the relevant issue numbers?

• gitlab-org/gitlab-ce#30548

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer