[BUGFIX] Allow fe_group modification when indexing subpage of restricted page

[deschilter]

What this pr does

When indexing a page, the authorized user is manipulated depending on which access is required to index.
Previously this did not take restrictions of parent pages into account when the page to index was not directly indexed.

How to test

1. Create a page called parent and restrict it to any fe_group
2. Check Extend to Subpages for the page parent
3. Create a subpage for the page parent called child
4. Index the pages

The subpage should be indexed now too.

Fixes #3969

[dkd-kaehm]

Thanks for fixing that issue.