Add multi-tenant support (add support for multiple research topics/subjects, per team)

[brunoamaral]

related to #340 , the objective is to enable us to track different subjects/topics of research with Gregory.

requirements

• each subject has its own set of categories
• an article can be in one or more subjects
• a subject belongs to a team

Note

Subject is a synonym for topic in this context.

Diagram for Database

Relevant code

https://github.com/brunoamaral/gregory/blob/main/django/gregory/models.py

[brunoamaral]

this was done in #387