Added CSRF to the application (took in account backwards compatibility)

Mitigated the XSS vulnerabilities reported by HackDefense
Advisories for said vulnerabilities can be found here:
https://hackdefense.com/publications/cve-2019-18345-davical-caldav-server-vulnerability
https://hackdefense.com/publications/cve-2019-18346-davical-caldav-server-vulnerability
https://hackdefense.com/publications/cve-2019-18347-davical-caldav-server-vulnerability