Notification sending component with persistence

[applejag]

After #33 is deployed and tested, we then need to consider persistence.

Add persistence of messages that are going to be send with email notifications. Messages should be first saved to the persistence and after that the module should attempt to send them. Messages should be deleted from persistence only after they are successfully sent.
We do not need to keep them in a database, yaml/json files should be enough.
Number of retries should be configurable (configmap from initial task). Exponential delay on the retries.