Fix "Review ID" column in the dashboard datagrid with a local site.

The review ID column would properly select which database field to access,
except DashboardDataGrid was popping the "local_site" value out of kwargs before
delegating to ReviewRequestDataGrid.

Testing done:
Created a local site and published a review request to it. Before this change, I
saw a review ID of "5" even though the URL was /s/test-site/r/1/. After this
change, saw a review ID of "1" in the dashboard.

Reviewed at http://reviews.reviewboard.org/r/3786/
Merge branch 'release-1.6.x' into release-1.7.x