API filtering improvements

[davidfischer]

This allows the API to be filtered in order to make it more usable. For example:

/api/v2/project/?slug=pip               # retrieve just the Pip project (matches slug exactly or not at all)
/api/v2/version/?project__slug=pip      # retrieve versions for Pip
/api/v2/build/?project__slug=pip        # retrieve builds for Pip

This is built on top of #4274 which documents the API. Only commits starting with e93d9ed need to be reviewed separately.

This reverses the changes in #2635 and #2903 and re-introduces django-filter. It uses some very limited filters where there are definitely indexes (slug) so performance should be decent.

[ericholscher]

Easy enough. Makes it much more useful though!