Add validations to admin settings (#10348)

* Add validations to admin settings

- Validate correct HTML markup
- Validate presence of contact username & e-mail
- Validate that all usernames are valid
- Validate that enums have expected values

* Fix code style issue

* Fix tests

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

@@ -2,6 +2,7 @@
   = t('admin.settings.title')
 
 = simple_form_for @admin_settings, url: admin_settings_path, html: { method: :patch } do |f|
+  = render 'shared/error_messages', object: @admin_settings
 
   .fields-group
     = f.input :site_title, wrapper: :with_label, label: t('admin.settings.site_title')