Allow/fix sending forms to new tab or window

Test Plan:
Ctrl+click on Show Diff in Chrome - button is not grayed-out, new tab is opened.
Click on it - button is grayed out.
Repeat in Firefox.

Reviewers: epriestley, tuomaspelkonen

Reviewed By: epriestley

CC: aran, Koolvin

Maniphest Tasks: T1137

Differential Revision: https://secure.phabricator.com/D2278