WIP authors: inital release

* Introduces models and fields for author records.

* Updates config file with new module name.

Signed-off-by: Mateusz Susik <mateusz.susik@cern.ch>

TO DO:
+ tests
+ discuss name -> previous_name field
+ discuss where should config variables go (there are more general than only for authors usage)
+ change config variables names

I don't think any of the variables conflict. What we could do as the convention inside of modules is to name variables starting with the module name, like ``AUTHORS_IDENTIFIERS_TYPES`` is if the config remains in this file to name them like ``AUTHORS_INSPIRE_YEAR_MIN`` instead of ``INSPIRE_YEAR_MIN``