Allow suppressing calls/messages from non-contacts

[mwild1]

Describe your feature proposal
The Android app has a setting to mute notifications from non-contacts, which can be useful to cut down on noise from e.g. spam calls.

A similar setting for iOS would be desirable.

Unlike the Android implementation, this will probably require server-side support to prevent sending of push notifications for non-contacts (or perhaps there is a way to send a silent notification?).