[TOTP] [UX] Improve usability on mobile/touch devices

1. I'd propose make the click areas bigger. They're too dense on mobile devices.
2. Optimise the Keyboard for number input

Click areas to small	Keyboard not optimised	Keyboard optimised for numbers (iOS app)

More information how to control the keyboard:

• http://bradfrost.com/blog/post/better-numerical-inputs-for-mobile-forms/
• Text Programming Guide for iOS: Configuring the Keyboard for Web Views

Server configuration

ownCloud version: 10.1.0 beta

List of activated apps:

root@owncloud:/var/www/owncloud# sudo -u www-data php ./occ app:list | grep twofactor
  - twofactor_backup_codes: 0.0.5
  - twofactor_totp: 0.5.2

Client configuration

Browser: SFAuthenticationSession in new iOS app

Operating system: iOs 12.1.3 (16D40)