Bug 10298: Provide a preference to enable mixed content rules.

This patch enables rules that trigger mixed content blocking if mixed content
blocking is disabled via security.mixed_content.block_active_content, or if
the pref extensions.https_everywhere.enable_mixed_rulesets is set.

Also add observers to reload the rules if either pref changes.
Merge remote-tracking branch 'mikep/bug10298-master'
Tweak and document Mike's change
Why indeed?