(LUN-1147) Links for pages on other sites are build correctly

If the page_link contained a page from a different site than the current site, the resulting URL was messed up, containing the absolute URL of the page and the domain of the current site. Fixed this by adding the correct domain name for a page's site that is different than the current one.
(LUN-1147) Made page_link a ForeignKey field

Reverted previous changes and changed the `page_link` field from `PageField` to `ForeignKey`. This change mainly occurs because of the requirement that the `page_link` should display only pages from the current site, so a `ForeignKey` field is more appropiate.
Merge pull request #34 from pbs/LUN-1147

(LUN-1147) Changed the page_link field from PageField to ForeignKey