Closes #6615: Enable filter lookups for custom fields

[jeremystretch]

Closes: #6615

• Refactors BaseFilterSet
• Ditches CustomFieldFilter in favor of stock filter intstances
• Add to_filter() method on CustomField model
• Moves MACAddressField form field to utilities (to resolve a circular import)
• Enables multi-value filters for custom fields

[jeremystretch]

I think this is mostly done, just need to fix a bug with integer handling and extend the test suite for custom field filters to test the lookups and multiple values.

[jeremystretch]

I think I've got this wrapped up, with just one issue: It seems that MultiValueNumberFilter isn't properly casting query parameters to integer values. This hasn't been a problem so far, because Django is smart enough to know that "10" means 10 when querying an integer field. However, custom field data is stored as JSON, so we need to be explicit about the type.

Here's what what the underlying SQL ends up as:

SELECT ••• FROM "dcim_site" WHERE ("dcim_site"."custom_field_data" -> 'foo') = '"10"'

As this is out of scope for this FR, I'm going to leave it be for the moment, and will file a new bug report.