Fix missing validation for MotDs

The backend wrongly trusted user input allowing the circumvention of
access control checks.

See merge request !37

The backend wrongly trusted user input allowing the circumvention of
access control checks.

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

The implementation (introduced by
2084f33a) was to permissive and posed a
security risk. As a consequence, cross origin requests requiring client
authentication are no longer possible.

Builds no longer depend on a preexisting config in ~/.ssh.