Add possibility to assign a pool number to a device

[xkilian]

Why:

To help our support teams we need to document what telephone number (or extension) is associated with which physical telephone (NetBox Device).

How:
Add to the plugin the ability for a pool to point to a DCIM/DEVICE.
Add a related table to appear in the Device model when a number/pool points to the device. (Show the following information: number, site, region, description, provider, forwards to.

When:
Like the other pool fields, make filling out this field optional.

[panaceya]

You can add your wishes to the code and make a pull request.