Theme Installer: Use JSONP for api.wordpress.org requests.

fixes #27639.


git-svn-id: https://develop.svn.wordpress.org/trunk@27961 602fd350-edb4-49c9-b593-d223f7449a82
Theme Installer: Don't encode description entities/HTML.

props johnbillion.
fixes #27641.


git-svn-id: https://develop.svn.wordpress.org/trunk@27962 602fd350-edb4-49c9-b593-d223f7449a82