Study and implement UI interface system notifications

[evilaliv3]

Since https://github.com/globaleaks/GlobaLeaks/issues/1015 depending on the configuration it happens that the UI prevents the user to see:

1. postopone log of internaltips
2. single receivers rtip unsubscription

this ticket deals with the refactoring of comments/messages in order to make them totally independent from action logging on itip and to the study and implementation of an UI interface system notifications.

defect reported by @2langnic