Editor scrolling:

* Make sure the background color for tools is only applied to the main editor.
* Fix toolbar width for smaller screens.
* Ensure toolbar doesn't end up in a weird place when scrolling down too fast.
* Avoid JS errors for post types that don't support the editor.

props avryl. see #28328.

Built from https://develop.svn.wordpress.org/trunk@29075


git-svn-id: http://core.svn.wordpress.org/trunk@28861 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Media Grid: decouple the Grid and Edit modal frames. Next steps: we shouldn't have to reload the frame while paginating.
See #24716.

Built from https://develop.svn.wordpress.org/trunk@29076


git-svn-id: http://core.svn.wordpress.org/trunk@28862 1a063a9b-81f0-0310-95a4-ce76da25c4cd