#31 API basic implementation

* adds new app to project
* adds relation between User model and new APIUserToken model
* adds first implementation for GET of intervention
* adds basic code layout for future extension by having new versions

user/models/user.py

@@ -15,6 +15,13 @@ from user.enums import UserNotificationEnum
 
 class User(AbstractUser):
     notifications = models.ManyToManyField("user.UserNotification", related_name="+", blank=True)
+    api_token = models.OneToOneField(
+        "api.APIUserToken",
+        blank=True,
+        null=True,
+        help_text="The user's API token",
+        on_delete=models.SET_NULL
+    )
 
     def is_notification_setting_set(self, notification_enum: UserNotificationEnum):
         return self.notifications.filter(