Fix conflict between normalize and array_attribute macros.

Fix the `normalize` and `array_attribute` macros conflicting with each
other on the WikiPage model. This meant code like
`wiki_page.other_names = "foo bar"` didn't work. Both macros defined a
`other_names=` method, but one method overrode the other.

The fix is to use anonymous modules and prepend so we can chain method
calls with super.

app/logical/concerns/array_attribute.rb

@@ -0,0 +1,36 @@
+module ArrayAttribute
+  extend ActiveSupport::Concern
+
+  class_methods do
+    # Defines `<attribute>_string`, `<attribute>_string=`, and `<attribute>=`
+    # methods for converting an array attribute to or from a string.
+    #
+    # The `<attribute>=` setter parses strings into an array using the
+    # `parse` regex. The resulting strings can be converted to another type
+    # with the `cast` option.
+    def array_attribute(name, parse: /[^[:space:]]+/, cast: :itself)
+      mod = Module.new do
+        define_method "#{name}_string" do
+          send(name).join(" ")
+        end
+
+        define_method "#{name}_string=" do |value|
+          raise ArgumentError, "#{name} must be a String" unless value.respond_to?(:to_str)
+          send("#{name}=", value)
+        end
+
+        define_method "#{name}=" do |value|
+          if value.respond_to?(:to_str)
+            super value.to_str.scan(parse).map(&cast)
+          elsif value.respond_to?(:to_a)
+            super value.to_a
+          else
+            raise ArgumentError, "#{name} must be a String or an Array"
+          end
+        end
+      end
+
+      prepend mod
+    end
+  end
+end