Datagrids have historically allowed for custom column selection/ordering

and sorting via HTTP GET query strings. This works great in the case

where you want to display a standard datagrid page with user choices,

but less so when you want to handle providing these options through

other means.
This change introduces new columns and sort arguments to the

DataGrid constructor. These both take lists of strings, and behave the

same way that the query string versions do. They can also take None,

which will force usage of the datagrid defaults.
This not only overrides (and disables) anything in the querystring, but

also prevents both loading from and saving to the user profile.

Unit tests pass.
Tested with some in-progress code using specially-constructed datagrids,

and verified I could define the columns and sort order.