Add browser notifications to mobile version

Many users complained about "not having an Android / iOS application" for diaspora*. When discussing with them, it looks like the main feature they are missing is notifications. We added them using pulling in the desktop version in https://github.com/diaspora/diaspora/pull/6952, this should be done in the mobile version too. It can be done by pulling at the moment, as that's the easiest way. Then there could be improvement, see https://github.com/diaspora/diaspora/issues/6668 for example.