Don't even load mixpanel when we're not tracking

This is better for privacy (mixpanel doesn't even see that this user is
loading the client).
The app should still work when Mixpanel is blocked

If Mixpanel is blocked (by the browser, a proxy, or just because it's
down) the app would lock during login, requiring a reload (if
stay-logged-in-across-reloads was on) to use.  This change fixes that.
Merge pull request #1198 from singpolyma/mixpanel-tweaks

The app should still work when Mixpanel is blocked