Added support for an activation webpage and renamed dialog config param
for consistency.

With this configuration both
 - spring security setup
 - bean properties
will be set to values defined in /etc/arsnova/arsnova.properties.

Admins ARE REQUIERED to create production settings file by given example
in /etc/arsnova.

This MUST be done because local developer settings ARE different to
the given example settings.

Deployed settings MUST be prepared by build process to match production
environment.

DO NOT expect local example settings will match all production settings.

This reverts commit 04bd8274.

switch to oauth2 authentication which allows facebook, twitter, google
etc authentication

redirection

include config.properties.example and let override values by
config.properties
cas spring security configuration adjustments 

* Added session controller with method to get session by id
* Created session class to be used as entity object
* Created session service class and interface
* Added content negotiation view resolver to map entity objects to JSON

This file should be used to create a config file with matching values.