Replaced PLCrashReporter with Crashlytics

props @sendhil
Removed some logging messages

Those were useful while developing, but right now they're just noise
Avoid initialising a navigation controller with a nil root
Merge branch 'crashlytics' into release/3.6.1
Use known URL when editing existing sites

Disallow editing URL/username for existing sites and use what's known
Fixes a problem when editing password on custom domains on
wordpress.com, since the SSL certificate won't be valid for the custom
domain
Changing some Mixpanel events into properties in order to optimize usage.