WIP: authors: initial release

* Initial module structure.
* Introduces SQLAlchemy models for Publication, Author and Signature.
* Introduces claiming operations on signatures.
* Adds citation association table.
* Tests for models.

Signed-off-by: Artem Tsikiridis <artem.tsikiridis@cern.ch>
Co-authored-by: Gilles Louppe <gilles.louppe@cern.ch>
Co-authored-by: Mateusz Susik <mateusz.susik@cern.ch>
Co-authored-by: Evangelos Tzemis <evangelos.tzemis@cern.ch>
@jirikuncar what we try to emphasize on is the separation of the record in the database ( ```invenio.modules.records.models.Record```) and the record returned by '''invenio.modules.records.api.get_record```. Hence, the renaming. There's no conflict, just for clarity. Is it unclear?