TOTP: Allow 'Enter' key to submit the auth code form

[r-a-y]

Currently for the TOTP method, if you have to add an authentication code for the QR code, you have to manually click on the Submit button in order for the form to submit properly.

This PR allows the Enter key to submit the auth code form, which is better for UX.

If you'd rather have the inline JS put into an external file, let me know. Inline JS was already used in the authentication_page() method so I followed suit.

Fixes #91.

[jeffpaul]

@kasparsd any thoughts on a review of this PR and whether it's ready for consideration in v0.8.0?

[jeffpaul]

Per yesterday's bug scrub, we're going to punt this to future release so that we can focus on the U2F deprecation in 0.8.0.