Review/Update servlet response headers

[MMaiero]

The security level of the UI can be improved updating the servlets response headers. In particular, the Ui can leverage the Content Security Policy feature implemented widely in most of the browsers (https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP)

GWT is not fully compliant with it. An investigation has to be performed to improve the configuration of the security-related headers. (https://github.com/gwtproject/gwt/issues/8197)

[MMaiero]

https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP

[MMaiero]

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control