Implement push notification simple interface

[charliemangano]

Description:

This is a subtask from #232. This subtask aims at implementing a fully functional push notification interface. The notifications should have an icon, a title and some details. We want to be able to send notifications to the current user as well as to a list of users.

Done:

• notifications have an icon, a title and a description
• notifications can be sent to the current user
• notifications can be sent to a list of users
• above 80% line coverage