Re-introduce data migrations

WHen re-creating the migrations during the merge, the data migrations for the admin user, default school and default school term have gone missing.

Please also merge !98 (merged) when working on this

added part::backend prio::1 labels

mentioned in merge request !111 (closed)

mentioned in issue #119 (closed)

removed milestone

assigned to @debdolph and unassigned @hansegucker

mentioned in merge request !112 (merged)

created merge request !112 (merged) to address this issue

mentioned in commit f6625580

closed via merge request !112 (merged)