sachet/server/models.py: no more url_for errors in CLI

url_for is now called when requested and not at user init. this means
that creating the user via the CLI doesn't complain about not being in
an active request anymore

sachet/server/models.py

@@ -89,9 +89,13 @@ class User(db.Model):
 
         self.password = self.gen_hash(password)
         self.username = username
-        self.url = url_for("users_blueprint.user_api", username=self.username)
         self.register_date = datetime.datetime.now()
 
+    @property
+    def url(self):
+        """URL linking to this resource."""
+        return url_for("users_blueprint.user_api", username=self.username)
+
     def gen_hash(self, psswd):
         """Generates a hash from a password."""
         return bcrypt.generate_password_hash(