Add a numeric input control for TOTP codes

Summary:
Fixes T11365. I tested these variants:

• <input type="number" />
• <input type="text" pattern="\d*" />

Of these, this one (using pattern) appears to have the best behavior: it shows the correct keyboard on iOS mobile and does nothing on desktops.

Using type="number" causes unwanted sub-controls to appear in desktop Safari, and a numbers + symbols keyboard to appear on iOS (presumably so users can type "." and "-" and maybe ",").

Test Plan: Tested variants in desktop browsers and iOS simulator, see here and T11365 for discussion.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11365

Differential Revision: https://secure.phabricator.com/D16323