Lifts some null constraints on item fields
Lifts one more item constraint on blank text
Removes most traces of Item#parent and #children

The "siblings" method still exists because it's needed to get projectless
items from the same user. Also, the "make tree" method remains commented out
as it may be needed down the line if I want to do model/collection based
javascript with it.

Also parent_id remains a column in the database for now.