Improve style for form in site settings (#2796)

app/javascript/styles/forms.scss

@@ -9,12 +9,6 @@ code {
   margin: 0 auto;
 }
 
-.admin {
-  input, textarea {
-    width: 100%;
-  }
-}
-
 .simple_form {
   .input {
     margin-bottom: 15px;

app/views/admin/settings/edit.html.haml

 - content_for :page_title do
   = t('admin.settings.title')
 
-= form_tag(admin_settings_path, method: :put) do
+= form_tag(admin_settings_path, method: :put, class: 'simple_form', style: 'max-width: 100%') do
   %table.table
     %thead
       %tr