BibAuthorID: single profile when moving signatures

* When a curator assigns a signature to a person
  these signatures can only be attributed to one
  pid. So far, one new pid was created per paper.

Signed-off-by: Artem Tsikiridis <artem.tsikiridis@cern.ch>
I believe the semantic here would change if you rewrite the if-statement like. I would rather do:
```
if 'pid_to_assign` in operation_parts:
    pid = operation_parts['pid_to_assign']
elif pid == bconfig.CREATE_NEW_PERSON:
    pid = create_new_person(uid)
```

(otherwise, because of your `else` clause, you will create a new person no matter what.)