bug: Enter keypress maps to 'back'

[pmespresso]

On form field componentMount, we should focus on the form field rather than the back button to prevent this behavior.

[ltfschoen]

I've been able to replicate the bug on these pages:

• Backup Account page - pressing "Enter" when page loads does nothing, but you're entering a value in the password input field and press "Enter" then it takes you back to the Account page
• Import account page - pressing "Enter" before clicking somewhere on the page (other than input field) takes user back to the Accounts page

The following pages do not appear to be impacted:

• Accounts page - pressing "Enter" does nothing
• Create a new account page - as above
• Account page - as above
• Add Tokens page - as above
• Send Ether or Send THIBCoin page - as above