Merge branch '732-reveal-contacts-ui' into 'master'

Add UI for revealing contacts within a private group

This addresses one part of #732. Join notices and onboarding will follow in separate MRs.

Although this MR is technically not based on !402, it does require it to be merged first to compile.

![device-2016-11-11-160454](/uploads/8cb6d8bdd0f3761657875024739e730a/device-2016-11-11-160454.png)

See merge request !406