Add Text model and supporting changes

• Add the Text model which belongs to a Passage and to an Edition. This also changes what the Passage model is: the Passage model no longer belongs to a particular Edition.
• Create and use the Contribution table as the many-to-many connection between Edition and Editor. This new table includes rank and contribution_type which allows us to get the first editor, have a separate editor and translator, etc.
• Create a Validation table and make each Text belong to a single validation, allowing each project to define their own validation system.

These changes are not final. I'd like to gather some feedback about these changes (I've left GitLab comments on the code) before merging.

Addresses https://framagit.org/anthologie-palatine/anthologyontology/-/issues/12 as well as some issues in https://framagit.org/anthologie-palatine/anthologyontology/-/issues/1