[572885] Disable external resources access when rendering SVGs

See CVE-2019-17566:

    Apache Batik is vulnerable to server-side request forgery, caused
    by improper input validation by the "xlink:href" attributes. By
    using a specially-crafted argument, an attacker could exploit this
    vulnerability to cause the underlying server to make arbitrary GET
    requests.

Completely disable access to external resources when rendering SVG
images, at least for now.

Bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=572885
See-also: https://nvd.nist.gov/vuln/detail/CVE-2019-17566
Change-Id: I2a1e9e618e2f2e0db1a3a6b362b9631492334dc0
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>