(NotificationsEmails) Notifications with emails

This is ready to be reviewed.

## Overview

### Notifications

Notifications impl. is based on activity streams, and uses an activity stream to represent a user's notification stream. Using internal node events, when new notifications are delivered to the feed, all users have "unread" counts that are incremented. There is also an endpoint where all of a user's notifications may be marked as read. The notification counts are tracked and incremented in redis, and when the counts change they are persisted into the user's basic profile. When a user has had notifications, they can be found through `/api/me`.

### Emails

A new `oae-email` module is there which can be used to send emails. Emails are sent by identifying a template by the module providing it and the "id" of the template. Email templates are internationalizable, and can be overridden per tenant using a configurable overrides directory.

Email templates, locale-specific templates and tenant overrides are provided using a directory and file-name convention, which is documented in the module documentation of `oae-email/lib/api.js`.