[DO NOT MERGE] Track login button clicks

How about this?
```
mdn.analytics.trackEvent({
  category: 'Sign-in',
  action: 'Click',
  label: serviceUsed.toLowerCase()
});
```
Basically, the default reporting hierarchy in GA is Category -> Action -> Label. But we can slice and dice event reports by any of those. E.g., if we want to see *all* 'Click' actions for some reason.

So long as the terms are semantically accurate and consistent (across GA and between GA & Optimizely), I'm happy.