We should be able to extract "back" handling, it now shows up in several locations:

```
change_password.js:      'click #back': 'back',
change_password.js:      'keyup #back': 'backOnEnter',
confirm_reset_password.js:      'click #back': 'back',
confirm_reset_password.js:      'keyup #back': 'backOnEnter'
delete_account.js:      'click #back': 'back',
delete_account.js:      'keyup #back': 'backOnEnter',
pp.js:      'click #fxa-pp-back': 'back',
pp.js:      'keyup #fxa-pp-back': 'backOnEnter'
tos.js:      'click #fxa-tos-back': 'back',
tos.js:      'keyup #fxa-tos-back': 'backOnEnter'
```