Force 2FA code input to show only numeric keyboard

Uses `inputmode` and `pattern` attributes on 2FA input
field so that it shows numeric-only keyboard on touch
screen devices for easier code input experience.

app/views/devise/sessions/two_factor.html.haml

@@ -8,7 +8,7 @@
           = f.hidden_field :remember_me, value: resource_params.fetch(:remember_me, 0)
           %div
             = f.label 'Two-Factor Authentication code', name: :otp_attempt
-            = f.text_field :otp_attempt, class: 'form-control', required: true, autofocus: true, autocomplete: 'off', title: 'This field is required.'
+            = f.text_field :otp_attempt, class: 'form-control', required: true, autofocus: true, autocomplete: 'off', title: 'This field is required.', inputmode: 'numeric', pattern: '[0-9]*'
             %p.form-text.text-muted.hint Enter the code from the two-factor app on your mobile device. If you've lost your device, you may enter one of your recovery codes.
             .prepend-top-20
               = f.submit "Verify code", class: "btn btn-success"