Move two classes to namespace of model using them

They are the only two classes using the namespace called Wikis
so far. We want to use this namespace for a module called `wikis` that
will encapsulate integration with external wikis and the internal OpenProject
wiki.

app/models/wiki_page.rb

@@ -161,7 +161,7 @@ class WikiPage < ApplicationRecord
     content_to = journals.find_by(version: version_to)
     content_from = journals.find_by(version: version_from)
 
-    content_to && content_from ? Wikis::Diff.new(content_to, content_from) : nil
+    content_to && content_from ? WikiPage::Diff.new(content_to, content_from) : nil
   end
 
   def version
@@ -171,7 +171,7 @@ class WikiPage < ApplicationRecord
   def annotate(compare_version = nil)
     compare_version = compare_version ? compare_version.to_i : version
     c = journals.find_by(version: compare_version)
-    c ? Wikis::Annotate.new(c) : nil
+    c ? WikiPage::Annotate.new(c) : nil
   end
 
   # Returns true if usr is allowed to edit the page, otherwise false

app/models/wiki_page/annotate.rb

@@ -28,7 +28,7 @@
 # See COPYRIGHT and LICENSE files for more details.
 #++
 
-class Wikis::Annotate
+class WikiPage::Annotate
   attr_reader :lines, :content
 
   def initialize(content)

app/models/wiki_page/diff.rb

@@ -28,7 +28,7 @@
 # See COPYRIGHT and LICENSE files for more details.
 #++
 
-class Wikis::Diff < Redmine::Helpers::Diff
+class WikiPage::Diff < Redmine::Helpers::Diff
   attr_reader :content_to, :content_from
 
   def initialize(content_to, content_from)