Root node parent_id null constraint

[andrewxhill]

Right now the JS engine requires that the root node not have a parent, but the presence of a parent is a good indicator that only a subtree of some larger tree is being displayed. Since the root node for display is being explicitly stated in the json object, I think we should remove the 'parent_id has to be null in the root node' constraint.

[sanderpick]

we can do this pretty fast by just axing the "ensure proper tree direction" in _Tree's _nest() which was originally intended for the re-rooting feature. It never gets executed with a standard render (in the current version).

[sanderpick]

they may be some snags here... I'll take a better look.

[andrewxhill]

There are alternatives if it isn't so straight forward. At the moment, when there is a subtree query, I swap out the parent_id of the new root node with null, so it would work as is, just would be incorrect in the grand scheme of the data.