Use more secure default for RelatedFilter.queryset

[rpkilby]

The secure default here is to simply remove the current default handling for the queryset argument. The underlying form field will complain about the lack of a queryset if it's not provided, so this forces the user to explicitly add it to the filter instance.

Ref #100. This is only a partial fix, as this only restricts the displayed choices for the relationship. This does not prevent against joins that might leak data.

This is a breaking change.

[codecov-io]

Current coverage is 97.73% (diff: 100%)

Merging #146 into master will increase coverage by 0.89%

@@             master       #146   diff @@
==========================================
  Files             6          6          
  Lines           222        221     -1   
  Methods           0          0          
  Messages          0          0          
  Branches         45         44     -1   
==========================================
+ Hits            215        216     +1   
+ Misses            3          2     -1   
+ Partials          4          3     -1   

Powered by Codecov. Last update ebb03cf...4126826