fix: sync langservers and extensions on any site config change
Created by: chrismwendt
This ensures that language Sourcegraph extensions are enabled/disabled to match global settings whenever the langservers
array in site config changes.
Not much of the logic around updating settings actually changed. All that changed was the call site and the place where the code lives.
Fixes https://github.com/sourcegraph/sourcegraph/issues/363
This PR does not need to update the CHANGELOG because it fixes an unreleased bug.