This adds a sign() and a verify() method to the CryptoComponent
that take a mandatory label argument to ensure that signatures can't be
repurposed.

and make sure work is offloaded to an executor, so exceptions can be
caught.

because they are not shown in the UI and are therefore not marked as
read. This fixes the unread message count.

This also adds two integration tests and improves some small details

Also react to incoming group invitations

Also close keyboard when returning from message fragment

interface and a factory for that.