Add a setting switch for js18n

We are getting 404 requests on the jsi18n url because the plumbing is
setup, but all of our translation is currently handled by server-side
code. This switch allows us to turn it off until needed.
allow testing server to override
Merge pull request #290 from ahinz/topic/flags-for-selenium

allow testing server to override
Merge pull request #288 from jwalgran/topic/jsi18n-switch

Add a setting switch for js18n