Set svg.context-properties.content.allowed-domains as empty

In Bug 1699892 (701d0e69), Mozilla introduced svg.context-properties.content.allowed-domains as a list of domains where non-standard, SVG context paint keywords are allowed. We should set this pref-list as empty.

Context paint is already allowed (and used) within Firefox, for local resources, but exposing it to web content opens a new vector of possible SVG-based exploitation.