Redesign of the signing process of the Onboarding Module

To improve usability, we have added a new screen to the signing process of the Onboarding Module. The new screen makes it easier for users to start a signing request, manage contracts within the signing request and get an overview of the whole signing process.
After starting a signing request, a header will appear with useful information about the transaction and the signing status. Of course, all existing functionalities have been retained.