Twenty Fifteen: Add left margin for lists inside blockquotes in `editor-style.css`.

Props diddledan.
Fixes #33380.
Built from https://develop.svn.wordpress.org/trunk@36075


git-svn-id: https://core.svn.wordpress.org/trunk@36040 1a063a9b-81f0-0310-95a4-ce76da25c4cd