Filtering cables by site slug or rack name does not work

[jeremystretch]

NetBox version

v3.1.0

Python version

3.8

Steps to Reproduce

1. Create a cable between two devices within a rack at a site
2. Attempt to filter the cables list by site slug (e.g. ?site=foo) or by rack name (e.g. ?rack=bar).

Expected Behavior

The cables list should return only cables which connect to a device within the specified site/rack.

Observed Behavior

The filters have no effect. This is because the site and rack filters are defined as MultiValueNumberFilters on CableFilterSet.