Add custom fields to all primary models

[jeremystretch]

Proposed Changes

Add support for custom fields to the following object types:

• dcim.Cable
• dcim.PowerPanel
• dcim.RackReservation
• dcim.VirtualChassis

Please note that this issue does not extend to any models not listed above: Custom fields are supported only for primary models at this time.

Justification

With #4878 completed, it makes sense to extend support for custom fields to all primary models. This has long been a design goal, but has been blocked by the need for improvements to the prior custom fields implementation.