Replaces #active_children with nested scopes #descendants.active
Removes calls to child_ids.
Makes parent parent assignment work on project form.
Use acts_as_nested_set instead of acts_as_tree for project hierarchy.
Adds a branch for unlimited project hierarchy.
CHANGELOG updated.
Merged r2109, r2118, r2130, r2131, r2134, r2135, r2136 and r2139 to r2145 from trunk.
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