PUSHY-99 - Send notifications to mobile notifications for new content in content API

Polls the content API for new content. For each new item, a notification will be sent to the mobile notifications API with a topic for each tag associated with the content.

Ids of items which we have received from the content API are persisted in a MongoDB collection to prevent sending notifications for the same item multiple times. This is because the content API provides no guarantees regarding the uniqueness of items returned when searching using the from-date filter. It appears to work on the webPublicationDate but the webPublicationDate can change. It changes for live blogs whenever a new post is added and on other content it can be manually changed by an editor. We therefore persist the item the first time we receive it and ignore it from then on.