Make Aphlict errors persistent.

Summary: Since an error is bad and means that some sort of action is required, make Aphlict error notifications persistent (i.e. they won't dismiss until being clicked).

Test Plan: Loaded a page with `notifications.enabled` set to `true` but without the Aphlict server actually running. Noticed that the error message didn't dismiss automatically.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9423