Fix a Firefox "scroll to bottom" bug when launching the media modal.

* Records main document scroll position when launching media modal.
* Restores position when media modal is closed.
* Also locks background document scrolling while media modal is open, preventing inadvertent scrolling there.

props koopersmith. fixes #22716

git-svn-id: https://core.svn.wordpress.org/trunk@23029 1a063a9b-81f0-0310-95a4-ce76da25c4cd