Also react to incoming group invitations

* Improve wording so contacts know they need meet up to scan
  and scan each other's codes
* Use consistent progress bar styles

All text that can be generated by users will show emoji from the
shipped sprites.

For all messages and posts, the custom emoji keyboard is now available.

This also disables menu actions rather than hiding them and thus
closes #677

Included is a fix for a regression that was not showing the keyboard
automatically in forums and thus
closes #676

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.

Closes #311

Fixes #539, #564 and #568.

Also properly handle back navigation when trying to add contacts.

Closes #342

Only personal blogs from non-contacts can be removed.
This also adds integration tests that check if blogs can actually be removed.

Closes #579

This refactors the forum invitation code,
so it can be used by both: forums and blogs.