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.

Conflicts:
	src/chrome/content/code/HTTPSRules.js
	src/components/https-everywhere.js
	src/defaults/preferences/preferences.js
This is going to be a pseudorelease tag, just for TBB
Merge remote-tracking branch 'mikep/bug10298-3.0' into 3.0
Changelog for 3.4.4tbb