Disable form on admin appearance sign-in page preview

The login form on the Admin Appearance preview page was previously
active. By disabling the form completely, it will be more clear
that this is just a preview and is not the sign-in form.

Changelog: fixed

app/views/admin/application_settings/appearances/preview_sign_in.html.haml

 = render 'devise/shared/tab_single', tab_title: _('Sign in preview')
 .login-box
   %form.gl-show-field-errors
+    - title = _('This form is disabled in preview')
     .form-group
       = label_tag :login
-      = text_field_tag :login, nil, class: "form-control gl-form-input top", title: _('Please provide your username or email address.')
+      = text_field_tag :login, nil, disabled: true, class: "form-control gl-form-input top", title: title
     .form-group
       = label_tag :password
-      = password_field_tag :password, nil, class: "form-control gl-form-input bottom", title: _('This field is required.')
+      = password_field_tag :password, nil, disabled: true, class: "form-control gl-form-input bottom", title: title
     .form-group
-      = button_tag _("Sign in"), class: "btn gl-button btn-confirm", type: "button"
+      = button_tag _("Sign in"), disabled: true, class: "btn gl-button btn-confirm", type: "button", title: title
 

locale/gitlab.pot

@@ -25306,9 +25306,6 @@ msgstr ""
 msgid "Please provide attributes to update"
 msgstr ""
 
-msgid "Please provide your username or email address."
-msgstr ""
-
 msgid "Please reach out if you have any questions and we'll be happy to assist."
 msgstr ""
 
@@ -34466,6 +34463,9 @@ msgstr ""
 msgid "This field is required."
 msgstr ""
 
+msgid "This form is disabled in preview"
+msgstr ""
+
 msgid "This group"
 msgstr ""
 

spec/features/admin/admin_appearance_spec.rb

@@ -34,6 +34,10 @@ RSpec.describe 'Admin Appearance' do
     visit admin_application_settings_appearances_path
     click_link "Sign-in page"
 
+    expect(find('#login')).to be_disabled
+    expect(find('#password')).to be_disabled
+    expect(find('button')).to be_disabled
+
     expect_custom_sign_in_appearance(appearance)
   end