Change validation for posts 'page' attribute.

issue #2305
- changed validation for 'page' to expect '0' or '1', rather than 'true' or
  'false'
- Added a 'can change a post to static page' test
- Added a 'can change a static page to a post' test
Merge pull request #2307 from Gudahtt/static-page-validation

Change validation for posts 'page' attribute