Add extra sign-in Tracks events

[frosty]

It'd be useful to track some additional behaviour around sign-in. Let's add some extra events:

• Sign in VC displayed
• Create account initiated
• Email already used (error) when creating a new account/user
• Username already exists (error) when creating a new account/user
• Failed to create a new account
• Number of signin screens viewed during the signin process (use a navigation delegate on NUXNavigationController to track pushes / pops?)

Android issue: wordpress-mobile/WordPress-Android#4903