[WIP] Contact Introductions

This MR allows you to introduce two of your contacts to each other. They both will receive an introduction with an optional message and then can accept or refuse the introduction which is presented as a notification.

When reviewing, I propose to review the individual commits separately as I took great care to split functional independent parts into separate commits. You might also want to have a look at the Introduction Client Wiki page to better understand what is going on before looking into the actual code.

I propose to postpone the re-introduction of the Dashboard that shows open introductions for another merge request.

This will finally close #118 (closed).