Allow a limit to be set on the number of results in a query panel

Summary: This allows a maximum number of items to be set in a query panel. Mostly useful when you have a query panel on the feed search and you don't want 4 billion results cluttering your dashboard.

Test Plan: Created a query panel with a maximum and it worked. Left it blank and got the default results.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T4980

Differential Revision: https://secure.phabricator.com/D9235