Add link custom field.

Summary: Ref T3868. Adds a link custom field that verifies and links to the text provided.

Test Plan: Add a custom field with type "link", then enter a well-formed URL in the provided box in the edit task screen. Verify that the URL displays correctly on the task display page.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T3868

Differential Revision: https://secure.phabricator.com/D12543