Fix inconsistent closing of bottom panel in Image Viewer

[LilyStilson]

The problem

Currently, if bottom pane contents in ImageViewerPage can scroll, then SingleChildScrollView overrides scrolling behavior of the Pane and there is no way to close it, unless you grab the header, which is not the part of a scroll view mentioned above.

Expected behavior

Panel closes regardless of its contents when user swipes down on it.

Device information

All platforms

Additional context

Looks like we'd need to wait for the SlidingUpPanel to be updated or find our own solution, because using provided ScrollController does fix the issue, but messes up pagination...