Fix XXE and XSS vulnerabilities

[bracketttc]

Apply recommended fixes from OWASP Cheat Sheets to attempt to fix XXE and XSS vulnerabilities in Valgrind Plugin. Disables external entities and escapes all content used in the reports for HTML using Apache Commons Text escapeHtml4.

Fixes SECURITY-1829, SECURITY-1830

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[earlruby]

I don't have the rights to merge this, but the changes LGTM.

[renatosilvagaio]

Who can merge this? It would be very useful.

[Fischmiep]

I would love to see this as well. I think @eXistence would be the correct person.

[renatosilvagaio]

Can someone generate a new version with these changes and make available in Jenkins plugins?
It would be much appreciated.
@eXistence