Project does not fully support musical work entities

[MaxFrax]

In many places in the project target_database.get_person_pid is used. Tough, only mix_n_match_client.py handles that maybe the entity could be a musical work. In which case target_database.get_work_pid is called.

My proposal is to merge the two getters into a single function which, given the entity type as input, returns the right PID. This can be achieved by moving mix_n_match_client.ENTITY_TYPES etc. into target_database.py.

[marfox]

Done, we should now fix the usage of target_database.get_person_id in:

• [x] ingestor;
• [x] validator;
• [x] linker.