[REF] models: use `parent_path` to implement parent_store

This replaces the former modified preorder tree traversal (MPTT) with the
fields `parent_left`/`parent_right`.  Each record is associated to a string
`parent_path`, that represents the path from its root node to itself.  The path
is made of the node ids suffixed with a slash:

              a                 node | id | parent_path
             / \                  a  | 42 | 42/
           ...  b                 b  | 63 | 42/63/
               / \                c  | 84 | 42/63/84/
              c   d               d  | 85 | 42/63/85/

This field provides an efficient implementation for parent_of/child_of queries:
the nodes in the subtree of record are the ones where `parent_path` starts with
the `parent_path` of record.  It is also more efficient to maintain than the
MPTT fields, and less sensitive to concurrent updates, because the value of
`parent_path` does not depend on sibling nodes.

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