Merge branch '357-introduction-feature-is-not-very-visible' into 'master'

Add first onboarding screen

When the user enters a private conversation after adding her second
contact, an onboarding screen will be shown highlighting the possibility
of introducing the contacts to each other.

![intro2](/uploads/b2cd4377f39974ca1fbd95649558e487/intro2.gif)

Closes #357

See merge request !324