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.

sessioninfo integration for sessionimport

This MR refactors the import of the public pool attributes for sessions.

See merge request !25

Convert database user IDs to lowercase on account creation an login.
Additionally, LDAP UIDs are lowercased as a fallback when they cannot be
retrieved from the server.