PostgreSQL: use UUID type and store tags in ARRAY not TEXT

Hi Mistral team,

In PostgreSQL, there is a UUID type, it should be used as it allows to store the UUID as a 16-byte array and not as a 36 character/byte string. SQLAlchemy is able to provide types that match the DB implementation (e.g. it would use String for SQLite and UUID for PostgreSQL).

The same applies to tags, they should be stored as an array of strings instead of a TEXT in PostgreSQL.

http://docs.sqlalchemy.org/en/latest/core/custom_types.html

Regards,
Xavier