Remove child labels of other label groups when they conflit

Closes #973 (closed)

What's in this MR?

When adding a child label (e.g. maintenance::pipelines), we will remove any other child label (e.g. bug::vulnerability) that is:

1. In the same label group
2. not from the same parent

Both conditions are ensured in the label_trees_in_same_label_groups method.

See the specs for some examples.