Add configurable CORS support

The original implementation did not correcly implement the W3C
Recommendation (https://www.w3.org/TR/2014/REC-cors-20140116/). CORS is
now implemented based the CorsFilter provided by Spring MVC.

Origins allowed for CORS can now be set in the configuration.
Additionally, GET access to the following request URLs is allowed from
all origins:

* /
* /arsnova-config and /configuration/
* /statistics