`fixTree` method

[adamjkb]

Solves problems that can appear when transactions are not used and a piece of code breaks, leaving the tree in an inconsistent state.
https://django-treebeard.readthedocs.io/en/latest/api.html#treebeard.models.Node.fix_tree

Not necessary for a basic implementation but it is certainly handy to have. Unfortunately, it cannot fix every issues but in return it's labour-intensive to implement.