fixes import error using latest version of FeinCMS

after the changes introduced in this commit
https://github.com/feincms/feincms/commit/bbc69a5a27b7d5902b3bb2f93aa098fbcbf53852
the import code breaks because PageAdmin can't be found in
feincms.module.page.models

test:
* ran django application with latest FeinCMS and old version of feincms-ext
* ran django application with latest FeinCMS and this code