Switch from the EasyList ruleset in AdBlock Plus format (as needed
by the old EphyURITester) to the JSON version, which can be directly
consumed by WebKitUserContentFilterStore.

Note that the EasyPrivacy ruleset is not available in JSON format,
and therefore its URI removed from the default list of rulesets.

This also renames the setting to "content-filters" in order to avoid
mixups with the old "adblock-filters" setting.