Add a data saver mode

[mplorentz]

It would be nice to have a toggle in our Settings menu (and maybe the Settings app as well) to put Planetary into a data/battery saving mode, where it would only gossip when the user pressed a button. This would dramatically reduce the amount of battery and data the app used for users with metered data or who are trying to conserve battery. We could even turn this mode on by default when the user puts their device in Low Power mode.

SSB discussion of this feature: %RY+HFs/w2ZadgqaRTmQOuBWGTbRk60b136SxTjGVQgI=.sha256