Show nanopubs from all keys of a user at the same time

[tkuhn]

Now that users can assign themselves multiple keys, it should be possible to show nanopubs signed by all these keys at the same time. Now, one has to switch from one to the other.

This can be achieved on the user interface side by replacing the radio buttons by checkboxes. On the server side, it's a bit more complicated as several queries will have to be issued at the same time.