Restore "Limit" to dashboard Query panels

Summary: See PHI1220. Ref T13272. I accidentally left the ability to set a query limit behind when updating this.

Test Plan: Edited a query panel, set/removed the limit, tried to set an invalid limit.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20472