Clear the session on setup start

The migration stores some values in the session, which causes a failed migration
to start from the point where it failed after a cleaned database. This can only
be fixed by clearing the session.
Rename projects with the same name under the same parent in migrations

We enforce unique project names under the same parent project since a few
versions via a database constraint.
fix the deletion handling and structure
stop migrating todos
stop migrating nodes
stop migrating filemanager entries
stop migrating contacts
stop migrating helpdesk entries
Stop migrating calendars

We just keep the module because we need some of the code, we don't really
provide a calendar any more.