Merge the new dashboard design into core.

Merges https://github.com/growthdesigner/wp-dash.

props lessbloat, joen, helen, dbernar1, kraftbj, ryelle, tillkruess, grapplerulrich, markjaquith.
see #25824.

Built from https://develop.svn.wordpress.org/trunk@26144


git-svn-id: http://core.svn.wordpress.org/trunk@26055 1a063a9b-81f0-0310-95a4-ce76da25c4cd