MOTD: Allow providers to send messages to the Bitmask users
We should give providers a way to send messages to Bitmask users using their services.
Alternatives:
- periodically poll a provider url, like https://provider.org/notifications
- add notifications to the @provider.json@ file and when the Bitmask client updates it the notification is displayed
Each notification need to have:
- message (multi language, same languages as supported on provider.json)
- expiration/validity... the users may not see this and after a while it may not make sense to show the message
- importance: low, normal high
Notifications will be displayed using #7601 (closed)
(from redmine: created on 2015-11-12, relates #7601 (closed), relates #7866 (closed), duplicates #2415 (closed))