Initial data model and migrations

The initial database model informed by the discussion in https://framagit.org/anthologie-palatine/anthologyontology/-/issues/1. The model is in anthology/api/models.py. The migration (anthology/api/migrations/0001_initial.py) is automatically generated by Django.

Some small notes:

• The main difference of this schema from the existing schemas is the Edition model. This model connects the work and passage.
• I used TextField instead of a CharField for text fields. This is because in PostgreSQL the two are identical and most other databases are moving in this direction.

Addresses https://framagit.org/anthologie-palatine/anthologyontology/-/issues/2 and (partially) https://framagit.org/anthologie-palatine/anthologyontology/-/issues/3