Topic mixpanel improvements. [Finishes #75444156]

Implemented new mixpanel tracking according to:
https://www.pivotaltracker.com/story/show/75444156

1. On registration box, if an error message displays (for example the user hasn't accepted the privacy policy, email field isn't filled in properly, password not long enough), then this should be recorded as 'Registraton unsuccessful'
under the 'Register' event
2. On the login box, if the user clicks on 'reset password', can this be tracked as 'Reset password click' under 'Login' event
3. If the user then goes ahead and enteres the email address to reset password, then can this be tracked as 'Reset password attempt' under 'Login' event
4. On Login box, if an error message is displayed because an incorrect email address is entered (for example user just inputs 'rebecca.ford'), can this be tracked as 'Login unsuccessful'