Use wide links instead of fake cursor in Differential

Summary:
Current approach has several problems:

• if there is no link in the cell then it still shows a link cursor
• if there is a link then it is clickable only on the text

Test Plan:
Display file in Differential, hover over cell with link.
Repeat for Paste.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1701