fixed loading sqlite helper in assignment screen

made assignment overview a table

I'm not too happy about the links for the delete action, but with the
form wrapped around the table, this was the easiest way.

fixed bugs introduced by making admin components

also show a TOC on both admin pages to easily switch

moved more assignment related stuff to the Assignment class

fixed getMenuText method signature

Add an adminpage for schema<->page/ns assignments

Todo: The test if a page is in a namespace with an schema assigned is
not yet implemented.