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.

Add functionality to save and restore recently used Emojis

Update emoji and add new categories based on AOSP's XML file

No more general disabling of abortOnError
Resolved all *errors* beside language ones  MissingTranslation & ImpliedQuantity
Issue context: #567

* Implemented in briar-core as a `ScheduledExecutorService`
  that gets started when the app starts
* The briar-api has a `FeedManager` interface
  that the UI can use to register and unregister feeds
* In this first iteration, feeds are fetched via HTTP(S), not Tor

Closes #484

Closes #515

Will currently fail at runtime; requires a public key and a server onion.

Modified the project structure, removed module extension and went instead for a non-complete core dependency graph

This commit renames identicons to avatars in field names, and uses an ImageView
derivative to show the identicons, which should make implementing #214 easier.