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>
> Everybody across Invenio knows what she/he can do with models.

That's a very strong assumption. IMO if the column isn't used outside the model, the underscore feels very natural. If the underscore is forbidden, then probably there should be a comment on `id_bibrec` field with a warning.