Convert InventoryItem's parent field to an MPTTField

[jeremystretch]

Issue type

[x] Feature request
[ ] Bug report
[ ] Documentation

Environment

• Python version: 3.5.2
• NetBox version: 2.2.8

Description

The parent field of an InventoryItem is used to arrange items hierarchically within a Device. For example:

- FPC 0
    - PIC 0
        - Xvcr 0

Currently, the parent field is a recursive ForeignKey, which does not allow for intelligent hierarchical ordering. We can change this to an MPTTField (like on the Region model) to greatly improve ordering and allow for more efficient queries.