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

src/applications/auth/factor/PhabricatorTOTPAuthFactor.php

@@ -154,6 +154,7 @@ final class PhabricatorTOTPAuthFactor extends PhabricatorAuthFactor {
       id(new PHUIFormNumberControl())
         ->setName($this->getParameterName($config, 'totpcode'))
         ->setLabel(pht('App Code'))
+        ->setDisableAutocomplete(true)
         ->setCaption(pht('Factor Name: %s', $config->getFactorName()))
         ->setValue(idx($validation_result, 'value'))
         ->setError(idx($validation_result, 'error', true)));

src/view/form/control/PHUIFormNumberControl.php

@@ -2,11 +2,28 @@
 
 final class PHUIFormNumberControl extends AphrontFormControl {
 
+  private $disableAutocomplete;
+
+  public function setDisableAutocomplete($disable_autocomplete) {
+    $this->disableAutocomplete = $disable_autocomplete;
+    return $this;
+  }
+
+  public function getDisableAutocomplete() {
+    return $this->disableAutocomplete;
+  }
+
   protected function getCustomControlClass() {
     return 'phui-form-number';
   }
 
   protected function renderInput() {
+    if ($this->getDisableAutocomplete()) {
+      $autocomplete = 'off';
+    } else {
+      $autocomplete = null;
+    }
+
     return javelin_tag(
       'input',
       array(
@@ -15,6 +32,7 @@ final class PHUIFormNumberControl extends AphrontFormControl {
         'name' => $this->getName(),
         'value' => $this->getValue(),
         'disabled' => $this->getDisabled() ? 'disabled' : null,
+        'autocomplete' => $autocomplete,
         'id' => $this->getID(),
       ));
   }