This adds a distinct option for ReviewRequest.objects._query and all

wrapping functions, allowing callers to query review requests without

distinct. When distinct is turned on (which is still the default for

now), it can impact how queries are built, breaking .count() for

instance.
A future version of Review Board is likely to switch the default for

distinct to False, but this will require further testing.
The _query() function was given type hints in the process, to help

with the implementation of the function. Work has not been done on the

wrapping functions to provide type hints.

Unit tests pass.
Tested this with some in-progress code to simplify queries for the

datagrid pagination.