add route to trigger Populate

+ use instance keys to stop multiple instances of populate
+ move Populate error to a new file
+ misc

app/lib/errors.py

@@ -0,0 +1,7 @@
+class PopulateCancelledError(Exception):
+    """
+    Raised when the instance key of a looping function called
+    inside Populate is changed.
+    """
+
+    pass