fix dangling links on homepage
allow user to delete collection

This deletes the collection from the db, deleting files is not
implemented yet.
mark collections for deletion by delete field
delete possible zombie scl when creating new

Allows user to recreate collection with same name after deleting it.
Otherwise it would lead to integrity error.
command to delete collections marked for deletion
cronjob for deleting files
Merge pull request #7 from msimacek/master

Allow user to delete his collection