Add way to notify admins when unapproved shelter updates are submitted

[marykuris]

When guest users or non-admin users submit updates to shelter entries, there is now no way to tell that the Shelters Update Queue has new entries. Admin users now need to manually check the queue.

This notification could use the same style as the existing Reload element.

It would be cool to add a notification to the Slack #shelters channel, as well.

[roozbeh]

PR created to address: https://github.com/Irma-Response/irma-api/pull/19