[Feature] Implement notification service

[vcasadei]

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like
In order to promote better and more consistent interaction with the app, it's necessary to generate app notifications calling the user to complete challenges.

Describe alternatives you've considered
These notifications should be sent one or more times a day with helpful tips, news and call for actions.
Also, there should be a place in which the user should be able to enable/disable notifications and/or change the frequency of notifications.

Additional context
We do not have design mockups yet.