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.


git-svn-id: https://develop.svn.wordpress.org/trunk@26144 602fd350-edb4-49c9-b593-d223f7449a82