Sort order SQL injection via `direction` parameter in administrate

In Administrate (rubygem) before version 0.13.0, when sorting by attributes
on a dashboard, the direction parameter was not validated before being
interpolated into the SQL query.

This could present a SQL injection if the attacker were able to modify the
direction parameter and bypass ActiveRecord SQL protections.

Whilst this does have a high-impact, to exploit this you need access to the
Administrate dashboards, which should generally be behind authentication.