[bug] Deal with notification storm

[nemesifier]

Related to #132
When an important event happens in the network (eg: blackout) and notifications are generated for hundred of devices, the following problems happen:

• the screen fills with toast notifications which seem to be infinite
• the sound keeps buzzing
• trying to open the notification widget fails because the UI is reloaded every second so it always shows loading

We should deal with these kind of events in a way that does not overwhelm users who have the UI open and are checking the situation, they know something big is happening and they do not need to be alerted for every notification. They also want to be able to check the notification widget properly (now it gets reloaded whenever a new notification comes up so the loading indicator shows up instead of notifications pretty much every second).