Rework related filtering

[rpkilby]

Alternative implementation to #197. This also needs further testing, but in general I'm happier with it.

The major difference is how data is passed between filtersets. In the current implementation (and #197), related filtersets are provided with a transformed data dictionary where their relationship name has been stripped from the data keys. The downside to #197 is that it's not possible to render related filterset forms.

In contrast, this approach passes the same incoming data to its related filtersets, untransformed. As a result, users should have a way to render related filterset forms with no conflict.

Overall, here is what this PR would enable:

• Correct behavior when filtering across to-many relationships (ref).
• Related Filters can now enforce queryset/choices restrictions. (eg, only display choices for related articles that have been published or are authored by the request user).
• No longer necessary to customize method filters (previously requires munging the field_name).
• Filtering against related annotations. Previously this was not possible, as the related filters were applied to the root queryset, and not the nested/related queryset.

Changes:

• Add FilterSet.relationship, which allows the filterset to parse it's params across relationships. This is compatible with form_prefix, although that's not usually relevant with API filtersets. It turns out, form prefixes aren't directly compatible, as the subsetting happens before parent initialization/filter deepcopy'ing, and prefix/relationship detection only occurs after. This could be supported, but it doesn't seem worth the trouble. i.e., we don't need prefixes to support rendering related filtersets.
• Add the FilterSet.get_related_filtersets() method and it's corresponding FilterSet.related_filtersets attribute. The latter is simply the cached result of the former. This method constructs a dict of related filtersets, keyed by their relationship name.
• Add the FilterSet.filter_related_filtersets() method, which is a hook for defining the behavior of how related filtersets are filtered.
• Add templates that display the first level of related filtersets.

TODO:

• Test FilterSet.get_related_filtersets()
• Test FilterSet.filter_related_filtersets() Implicit through existing tests
• Test templates (test post data)
• Test related filterset errors
• Ensure there are no tests marked as expected failures

[codecov-io]

Codecov Report

Merging #199 into master will increase coverage by 0.42%.
The diff coverage is 100%.

[rpkilby]

Alright, I'm largely satisfied with this PR. I need to carefully review the code and ensure the tests adequately cover the new behavior, but otherwise, this PR accomplishes what I want it to.

In addition to the general improvements to filtering across relationships, it's worth noting that this PR enables annotation support for related filtersets, as well as form rendering for first-level relationships.