* Reduce the starting height of the editor to better match the height at which the top toolbar unpins.
* Pin the side metaboxes so they do not get lost when the editor content is long.
* Turn magic numbers into variables.
props stephdau, azaozz. see #28328.
Built from https://develop.svn.wordpress.org/trunk@29495
git-svn-id: http://core.svn.wordpress.org/trunk@29273 1a063a9b-81f0-0310-95a4-ce76da25c4cd
