Bug 1024507: Measure use of Twitter and Facebook

<strong><em>IMPORTANT:</em></strong> After merging this, please remind me to update the client URL of my Facebook application. At the moment, to aid in testing, the application accepts calls from developer-local.allizom.org. Changing this takes no time, so if this merges while I'm on PTO feel free to just shoot me a text.
Good question. Yeah, we might. If we want this to work on all of our servers, we would need to set up an application for each server. That would make my point in the first comment irrelevant, too.

I'll take a look.