Don't prompt to mark notifications as read if we don't need to

Summary: Fixes whatever task is tracking this junk, if one exists. Don't prompt unless there's a security issue.

Test Plan:

• Generated notifications from a test account.
• Clicked "Mark All" from dropdown menu, no prompt.
• Clicked "Mark All" from notifications screen, no prompt.
• Command-Clicked "Mark All" from dropdown menu to open in new window, got normal prompt.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D18483