Implement E-Mail notifications

[OLED1]

There should be notifications sent, when:

• a service become critical (e.g. died, not reachable, etc.)
• a service become ok again(e.g. reconnected, etc.)
• the wallet has received some Mojo's or XCH
• ...

This notification setting should be able to be enabled in the system settings where default rules (in which case should be alerted or informed) can be applied in the system settings page and be configurable per user base in the Personal Settings.