wikis: normalize Unicode characters in wiki bodies.

* Introduce an abstraction for normalizing attributes. Very loosely
  modeled after https://github.com/fnando/normalize_attributes.
* Normalize wiki bodies to Unicode NFC form.
* Normalize Unicode space characters in wiki bodies (strip zero width
  spaces, normalize line endings to CRLF, normalize Unicode spaces to
  ASCII spaces).
* Trim spaces from the start and end of wiki page bodies. This may cause
  wiki page diffs to show spaces being removed even when the user didn't
  explicitly remove the spaces themselves.

app/logical/concerns/normalizable.rb

@@ -0,0 +1,18 @@
+module Normalizable
+  extend ActiveSupport::Concern
+
+  class_methods do
+    def normalize(attribute, method_name)
+      define_method("#{attribute}=") do |value|
+        normalized_value = self.class.send(method_name, value)
+        super(normalized_value)
+      end
+    end
+
+    private
+
+    def normalize_text(text)
+      text.unicode_normalize(:nfc).normalize_whitespace.strip
+    end
+  end
+end