✨(models/api) add RBAC on templates linking accesses to a team name

We want to be able to control who can access a template via roles.
I added this feature on the TeamAccess model assuming that the teams
to which a user belongs can be retrieved via a `get_teams` method on
the user model. The idea is that this method will get the teams either
via a call to an external API or directly from the OIDC token upon
user login. This list of teams will probably have to be cached for
each user.

src/backend/core/tests/conftest.py

@@ -0,0 +1,15 @@
+"""Fixtures for tests in the publish core application"""
+from unittest import mock
+
+import pytest
+
+USER = "user"
+TEAM = "team"
+VIA = [USER, TEAM]
+
+
+@pytest.fixture
+def mock_user_get_teams():
+    """Mock for the "get_teams" method on the User model."""
+    with mock.patch("core.models.User.get_teams") as mock_get_teams:
+        yield mock_get_teams