Make it possible to use builtin avatars

Images are from openclipart.org

calsocial/models.py

@@ -285,6 +285,9 @@ class Profile(db.Model):  # pylint: disable=too-few-public-methods
     #: If locked, a profile cannot be followed without the owner’s consent
     locked = db.Column(db.Boolean(), default=False)
 
+    #: If set, the profile will display this builtin avatar
+    builtin_avatar = db.Column(db.String(length=40), nullable=True)
+
     @property
     def fqn(self):
         """The fully qualified name of the profile