The toolbar subtitle shows information about how many contacts the
current blog is shared with and how many of those are online.

* Remove distinction between "shared with" and "shared by"
* Show all contacts a blog is shared with
* Show online status of contacts in sharing screen

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.

This also adds two integration tests and improves some small details

Also react to incoming group invitations

interface and a factory for that.