Added mass submit/discard to dashboard

This commit adds a CheckboxColumn to the datagrid in the dashboard, as
well as some links in the datagrid header to submit or discard all the
selected reviews.  When clicked, the links will send API requests to
close each of the selected reviews and will then reload the page.

This code depends on the CheckboxColumn being added to djblets, which
I've submitted in another review: http://reviews.reviewboard.org/r/3796/

- Confirmed that submit/discard selected buttons closes the reviews then reloads the page
- Confirmed that check/uncheck all works