BUG Remove extraneous </div> breaking IE8 image embedding (os#8218)

Editor was not able to add images to TinyMCE - both newly uploaded and
old ones from the local assets.
Rewritten "extend cms" docs (#1671)

Hopefully this commit can be reverted once we fix the
layout manager to work with all four directions (north, south, east, west).
A "bookmark bar" makes more sense as an example than having the links
in the menu, and it allows us to illustrate the CMS layout techniques.
FIX ConfigManifest regenerating every request if variantKeySpec is an empty array()
Use httpError() instead of non-existent HTTPResponse_Exception class
Merge pull request #2207 from camspiers/config-caching-fix

FIX ConfigManifest regenerating every request if variantKeySpec is an empty array()
Merge branch '3.0' into 3.1
Merge branch '3.1'