The datagrids code is a hefty portion of reviewboard.reviews, and is

going to be heftier soon. Especially considering it was bundled into one

big file. It also felt strange to have extensions import from

reviewboard.reviews.datagrids for things that weren't related to review

requests.
This introduces reviewboard.datagrids, which contains all the columns,

datagrids, URL mappings, and views.
The datagrid and column code have been untouched, with a couple

exceptions. I rearranged the columns to be in alphabetical order,

renamed StarColumn to BaseStarColumn, added a missing localization for

the label for NewUpdatesColumn, and replaced double quotes with single

quotes. No logic has changed anywhere.

Unit tests passed.
Viewed every datagrid:

The Dashboard (Starred, Outgoing, a couple Incoming pages, and Watched Groups)
All Review Requests
Groups list
Review requests on a group
Submitters list
User page

I was able to view the datagrids and toggle Show/Hide Closed when available.
I didn't test adding columns to each one, but I tested a couple, and they worked. That code

is all common anyway.