Inconsistent constraints for parent disciplines

[timobrembeck]

Describe the Bug

At the moment, in the discipline form only disciplines without training sets can be selected as parent disciplines.
However, this rule is not applied when the hierarchy is changed via drag & drop, which leads to inconsistencies.

Steps to Reproduce

1. Go to disciplines
2. Move a discipline to another discipline which has training sets

Expected Behavior

Show an error message that this is not possible

Actual Behavior

The discipline is moved without problems and the parent is set in the database.
Only when the form is opened, the parent "looks" empty (even though it's not) and when the form is saved, the parent is empty again.

Additional Information

I'm not sure what the intention is behind this constraint, but nonetheless - it should be enforced consistently in both the form and the drag & drop feature.