FIX: Validate permalink_normalizations setting (#16604)

When an admin enters a badly formed regular expression in the
permalink_normalizations site setting, a RegexpError exception is
generated everytime a URL is normalized (see Permalink.normalize_url).

The new validator validates every regular expression present in the
setting value (delimited by '|').

lib/validators/regexp_list_validator.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class RegexpListValidator
+  def initialize(opts = {})
+  end
+
+  def valid_value?(value)
+    value.split("|").all? do |regexp|
+      begin
+        Regexp.new(regexp)
+      rescue RegexpError => e
+        @regexp = regexp
+        @error_message = e.message
+        false
+      end
+    end
+  end
+
+  def error_message
+    I18n.t("site_settings.errors.invalid_regex_with_message", regex: @regexp, message: @error_message)
+  end
+end