Nested Location Support

[DanSheps]

Moving https://github.com/tbotnz/netbox_floorplan/issues/5 here

Use case

Currently, we have our datacenter racks laid out in locations by:

• Floor
• Room
• Row
• (Optional) Functional Group

Unfortunately, you cannot include devices in nested locations within the "Room" plan (Rack in Row 1, Infrastructure for example cannot be included in the "Room drawing").

Proposal

There are two options to solve this problem, one or the other method could be used:

1. Allow devices from the location including all child locations
2. Allow placement of child drawings

### Tasks
- [ ] https://github.com/netbox-community/netbox-floorplan-plugin/issues/7
- [ ] https://github.com/netbox-community/netbox-floorplan-plugin/issues/8