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.

added a cache to the IdentityManager, changed its signature, modified when and where the author is stored

made the author creation single-threaded again in the LifecycleManager, removed redundant code

and turn them into a regular string.

Also removes support for anonymous forum posts.
Closes #698