Refs #6288; Changed the following:

- Added browser prefixes for moz and webkit when using box-sizing in
  order to avoid width differences within older browsers
- Added default color and IE9 svg gradient background for the
  readability gradient table
- Tweaked advanced readability window size in order to better fit on IE
- Load the eeareadabilitychecker.pt from
  portal_skins/eeareadabilitychecker since from the portal_url you get
  the frontpage when the template isn't customized
- Load the css from portal root allowing us to customize the resources
  without the need to edit the js file that loads them
Refs #6288; added history entry for latest changes