Adding order to database_options for MySQL report types

This PR adds the ability to order the results of a database query for a VariableHeader.

Adding an `order` key to the `database_options` object on a VariableHeader will sort the database options before creating the drilldown.  Accepted keys for `order` are `ASC` or `DESC`.

I wasn't sure how this would play with AdoDb, so I didn't modify that report type, but I imagine it would work just fine with a similar update.