Support association proxies in path when determining hybrid property

1) Added support for dealing with association proxies when determining if field path is a hybrid property.
2) Restored support for filters using column values (not just their names).

Fixes #1339 and adds python 3 support to the example

Fix CSRF for production deployments

Added support for remote hybrid properties filters

Current SecureForm implementation generates CSRF secret using
`os.urandom()` every time when application start up. CSRF secret is used
to calculate csrf_token check value, so if someone would use a command
similar to

    gunicorn --workers=8 app

to run his flask-admin app on production then most form submissions
would silently fail (silently - as for now, it is probably another one
bug).

Instead of custom `os.urandom()` logic the `app.secret_key` value should
be used to produce CSRF token values.

Added support for sqla association proxy in model view.

allow user to set  flag "ignore_hidden"

Fix geoa.GeoJSONField

support for WTForms >= 3.0

Updated hybrid property detection to new sqlalchemy API

Modified hybrid property example to make use of the scaffold mechanism to create filters.

support multiple column sort with column_sortable_list

Update actions.js

Fix url generation with localized named filters.

Create function to toggle row checkbox if all checkboxes were toggled manually

Support single table inheritance for inline models (sqla)

Enable support for after_model_ actions in appengine

Support form_args for appengine