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
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.
- Commands that run a truncate DB action failed due to messing up the
test transations so we mnaully work around that now to ensure a
transaction exists for the test to cleanup afterwards.
- Updated dompdf lib version
