(bug 42202) Validate preference values in action=options

(bug 42202) Validate preference values in action=options

Previously, there was no validation whatsoever and the module would
happily write any preference you asked it to. This, combined with the
fact that the code using the 'editfont' preference didn't perform any
validation or escaping, led to a CSS injection vulnerability.

Using Preferences::getPreferences breaks some existing test cases
because a MockUser doesn't have groups for preferences.

Change-Id: I98df55f2b16ac1b6fce578798b6f58b5dad96775