Add update notifications

closes #1464

- adds opt-out via updateCheck:false in config.js
- update check is done on admin index, but doesn't interfere with rendering
- adds update check module, which gets the usage data, makes the request and handles the response
- adds two new settings to default-settings, one for next check time, and one for whether to show the notification
- adds a new rejectError method to errorHandling
- adds a new helper for displaying the notification

Conflicts:
	core/server/helpers/index.js
	core/test/unit/server_helpers_index_spec.js