Merge pull request #88 from adrexia/siteconfig

BUG: Subsite save SiteConfig overwrites MainSite SiteConfig (Issue #15)
Change the preview iframe to use injected SubsiteID GET/POST params.

Use the CMS domain for fetching the preview content to avoid SSL
errors and single-origin violations on the iframe.
Merge pull request #90 from mateusz/previews-by-id

Change the preview iframe to use injected SubsiteID GET/POST params.
BUG Fixing "Add new" not adding the page to the correct subsite
Add custom menu-icon