Claim attribution fix

This changes the behaviour when a signature is rejected.

### What is happening so far

When a signature is rejected, a new profile is created with just this signature.

So if somebody sees that there are 5 different papers for John Ellis that should not be there, they get reassigned to 5 newly created profiles.

### What this PR is changing

When a signature is rejected, it is actually reassigned to somebody with the exact matchable name who has not rejected this paper. If there is none, a new profile is created and subsequently all the signatures go there.

### Why this is a WIP

Although the implementation of this change (along with a bug fix :) ) is done, we would like your input to understand if we really really want this.

The idea has been, that a signature gets reassigned somewhere (a new profile everytime ? ) and then tortoise is supposed to fix it. However, since in reality tortoise is not running unsupervised at the moment, we just rely on the curators.

Now, each signature tries to get reassigned to an existing profile. So here is the trade-off:

* No more one-signature profiles!
* a signature may be reassigned to a an existing profile when it shouldn't be! Of course the author can say "it is not mine!"  and then it will be moved again. This can make an already self-curated profile "dirty", unless the owner provides input like it "is not mine".

Which behaviour is desirable ?

cc @glouppe 
After talking with @atsikiridis we came to the conclusion that this is the safest solution (rather than changing `move_signature` itself, on which other parts of the code depend).