Add description field to pass-through ports

[jeremystretch]

Environment

• Python version: 3.5.2
• NetBox version: v2.5-beta1

Proposed Functionality

Add a description field to the FrontPort and RearPort models.

Use Case

A description field could be very useful when modeling things other than simple patch panels (e.g. a network tap).

Database Changes

Add the following field to both models:

    description = models.CharField(
        max_length=100,
        blank=True
    )

External Dependencies

None