[RelDB] Remaining minor todos

[luisa-beerboom]

• database_reader.py has a function get_current_migration_index that currently always returns -1. It is only called in the database-writer.py where the line is commented out. Fix or remove for cleanup.

Test related

• agenda_item/test_assign.py in assign_parent_set test there seem to be far more datastore calls than before.
• unit/test_presenter.py: write more unit tests
• meeting/test_update.py: test_update_cant_lock_template_2 test should be a db constraint