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
- Moved thumnbail loading out of repo into ImageResizer.
- Updated gallery and editor image handling to show errors where
possible to indicate memory issues for resizing/thumbs.
- Updated gallery to load image data in a per-image basis via edit form
for more resiliant thumb/data fetching. Data was previously provided
via gallery listing, which could be affected by failing generation
of other images.
- Updated image manager double click handling to be more pleasant and
not flash away the edit form.
- Updated editor handlers to use main URL when thumbs fail to load.
