HTML rendering for filters

[lovelydinosaur]

Closes #2180.

Filters rendering in the browsable API and admin.

Remaining:

• Docs.
• Use translated strings throughout.
• Render bootstrap forms directly, dropping the crispy forms optional req.
• Consider search control rendering directly? (But could be a second pass)

This sort of thing...

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[jpadilla]

@tomchristie looking great, any specific reason for introducing the requirement for crispy forms?

[lovelydinosaur]

@jpadilla django-filters won't render nicely with our bootstrap style out-of-the-box.

We'd probably be making it a soft requirement, but there's also a poss of moving DjangoFilterBackend into a third party package, and then including a built-in field filtering class that uses serializers (since they actually have everything we'd need to make this work).

[carltongibson]

there's also a poss of moving DjangoFilterBackend into a third party package

I'd be happy to maintain such a package.

I have a slight niggle about re-creating functionality that already exists; the prejudice is to consolidate effort rather than split it...

[lovelydinosaur]

I'd be happy to maintain such a package.

That would be pretty amazing. Ideally I'd like to see us do that with both django-filter and django-guardian so that external deps are only required in external packages.

[carltongibson]

OK. Fine. I'll put something together.