Added configurable spam checks and minor changes

[FEATURE] Added extendable spam check for registration form #695
[BUGFIX] Fixes wrong validation behavior with reCAPTCHA #694
[TASK] Change registration of longitude- and latitude-evaluators #696
[TASK] Updated license in composer.json
[TASK] Use f:link.action viewHelper in Calendar.html view
[TASK] pass "failed" status variable to the template
[DOC] Added documentation for extendable spam check #695
[DOC] Added documentation for XML sitemap configuration #677