Removes the `.customize-partial-edit-shortcut-left-margin` class which was not effective on some themes, created a worse experience for other themes, and which did not recalculate when the preview was reflowed or resized. Now some small-width media queries are added to handle common cases while more dramatic issues can be handled by the theme. Also renames `Partial.positionEditShortcut()`to `Partial.addEditShortcutToPlacement()` which is a more accurate description of its function.
Props sirbrillig, sstoqnov.
See #38651, #27403.
Built from https://develop.svn.wordpress.org/trunk@39202
git-svn-id: http://core.svn.wordpress.org/trunk@39142 1a063a9b-81f0-0310-95a4-ce76da25c4cd
2185609a33