Add 'autocomplete="off"' to MFA TOTP inputs

Summary:
Ref T13202. See https://discourse.phabricator-community.org/t/2fa-input-box-isnt-hinted-as-a-password-so-browsers-suggest-auto-fills/1959.

If browsers are autofilling this, I think browser behavior here is bad, but behavior is probably better on the balance if we hint this as autocomplete="off" and this is a minor concesssion.

Test Plan:

• I couldn't immediately get any browser to try to autofill this field (perhaps I've disabled autofill, or just not enabled it aggressively?), but this change didn't break anything.
• After the change, answered a TOTP prompt normally.
• After the change, inspected page content and saw autocomplete="off" on the <input /> node.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13202

Differential Revision: https://secure.phabricator.com/D19722