Support models with primary keys not named "id"

[sterliakov]

Fixes #16.

Django models do not always have a field named "id". Declaring another field with primary_key=True removes the default id field.

Admin *ListFilter variations all have lookup_kwarg attribute that provides exactly what we need: explicit target lookup without any string munging in application code. This also allows us to remove isChoiceField entirely and always use the lookup_kwarg provided by Django. Autocomplete AJAX always returns PK field under id field.

I tested these changes locally on a test project and adjusted the existing test to account for new fields. To support usual whitespace wrapping, I replaced string-based asserts with HTMLParser (built-in) to verify that there is exactly one tag matching given specification.

[sterliakov]

Without this change, editing HTML files does not trigger pytest - probably not what you need?

[vigo]

@sterliakov buddy, great work, i can not merge it, your commits are not signed. can you please sign your commits? i'm super exited to merge it.

[sterliakov]

Done, but please consider adding this to CONTRIBUTING.md - signing commits is an unusual requirement (and often adds no actual security: https://softwareengineering.stackexchange.com/questions/212192/what-are-the-advantages-and-disadvantages-of-cryptographically-signing-commits-a)

[vigo]

You are right, i should add it to CONTRIBUTING.md, thank you for the great support <3