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

app/models/application_record.rb

@@ -3,6 +3,7 @@ class ApplicationRecord < ActiveRecord::Base
 
   include Deletable
   include Mentionable
+  include Normalizable
   extend HasBitFlags
   extend Searchable
 

app/models/wiki_page.rb

@@ -3,11 +3,13 @@ class WikiPage < ApplicationRecord
 
   META_WIKIS = ["list_of_", "tag_group:", "pool_group:", "howto:", "about:", "help:", "template:"]
 
-  before_validation :normalize_title
   before_validation :normalize_other_names
   before_save :update_dtext_links, if: :dtext_links_changed?
   after_save :create_version
 
+  normalize :title, :normalize_title
+  normalize :body, :normalize_text
+
   validates :title, tag_name: true, presence: true, uniqueness: true, if: :title_changed?
   validates :body, presence: true, unless: -> { is_deleted? || other_names.present? }
   validate :validate_rename
@@ -151,10 +153,6 @@ class WikiPage < ApplicationRecord
     title.to_s.downcase.delete_prefix("~").gsub(/[[:space:]]+/, "_").gsub(/__/, "_").gsub(/\A_|_\z/, "")
   end
 
-  def normalize_title
-    self.title = WikiPage.normalize_title(title)
-  end
-
   def normalize_other_names
     self.other_names = other_names.map { |name| WikiPage.normalize_other_name(name) }.uniq
   end