Make layers in a hierarchy the same way categories can be nested

[jetherton]

Categories can be in levels 2 deep. They want N levels of depth for Layers.

This means:

• Adding a parent_id field in the layer table
• reworking the UI on the layers edit page to assign parent layers to things
• Probably need to think about making parent levels not really layers, but just placeholders for the kids.
• Change the layers UI on the main page

[jetherton]

Left todo:

• Switch to nested arrays instead of hitting the DB for each level of nestedness.
• Don't let a user be able to select themselves as their child, grand child, great grandchild....
• Add code to check for circular dependencies just in case.