Added HTML validation
See #3619
The node dependency can be swtiched to npm when the PR https://github.com/jzaefferer/grunt-html/pull/21 is merged
Validate the unminified HTML files
Added html validation custom task and ignores for ajax files
Added html validation ignore for new HTML5 tags
Moved some more html files to the ajax validation task
Fixed most HTML validation errors
Ignore html validation errors about longdesc