Adds validation that the related page exists, is visible, and that the
user has page edit permissions for the page.
Aligns with attachment permission model, and aligns across different
image endpoints.
Added tests to cover.
Closes#6126
Had misalignment between query and usercan, The nuance between fallback
and entity-role permissions was not taken into account by the query
system. Now added with new test cases to cover.
Ported non-compatible elements, Now all tests passing apart from some
specific permission scenario tests which are probably correctly failing.
Updates some tests to better avoid messing environment state.
