FIX Opt-out pf form message escaping (fixes #2796)

This fixes a limitation introduced through http://www.silverstripe.org/ss-2013-008-xss-in-numericfield-validation/.
Form messages used to accept HTML, now they’re escaped by default, effectively removing the ability
to pass in HTML and take care of escaping manually.

We pass through HTML to message in core through the CTF system, so this needs to be fixed.
It’s an alternative fix to https://github.com/silverstripe/silverstripe-framework/pull/2803.