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/validators/html_validator.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class HtmlValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    return if value.blank?
+    record.errors.add(attribute, I18n.t('html_validator.invalid_markup')) unless valid_html?(value)
+  end
+
+  private
+
+  def valid_html?(str)
+    Nokogiri::HTML.fragment(str).to_s == str
+  end
+end