Make the asset tree more flexible by changing Section to self referencing

[Gavin]

This is a very good idea and would make the asset tree depth that extra bit more flexible.

Example:

• Company
  • Region
    • Site
      • Section

Company and Region are currently not catered for, while Section already has a Department.

This affects:

• Database schema.
• Asset tree taglib as accessed from the tree icon on the main nav.
• The asset tree import and export functions.
• Cascase deletion and updates.

Hence placing in future milestone for now.