Move serializers pagination class to separate module

This helps to avoid conflicts with `Paginator` class that seems to be
used by some bundled libs.

app/serializers/environment_serializer.rb

@@ -8,7 +8,7 @@ class EnvironmentSerializer < BaseSerializer
   end
 
   def with_pagination(request, response)
-    tap { @paginator = Paginator.new(request, response) }
+    tap { @paginator = Gitlab::Serializer::Pagination.new(request, response) }
   end
 
   def itemized?

app/serializers/paginator.rb

-class Paginator
-  include API::Helpers::Pagination
-
-  def initialize(request, response)
-    @request = request
-    @response = response
-  end
-
-  private
-
-  # Methods needed by `API::Helpers::Pagination`
-  #
-
-  attr_reader :request
-
-  def params
-    @request.query_parameters
-  end
-
-  def header(header, value)
-    @response.headers[header] = value
-  end
-end

app/serializers/pipeline_serializer.rb

@@ -4,7 +4,7 @@ class PipelineSerializer < BaseSerializer
   entity PipelineEntity
 
   def with_pagination(request, response)
-    tap { @paginator = Paginator.new(request, response) }
+    tap { @paginator = Gitlab::Serializer::Pagination.new(request, response) }
   end
 
   def paginated?

lib/gitlab/serializer/pagination.rb

+module Gitlab
+  module Serializer
+    class Pagination
+      include ::API::Helpers::Pagination
+
+      def initialize(request, response)
+        @request = request
+        @response = response
+      end
+
+      private
+
+      # Methods needed by `API::Helpers::Pagination`
+      #
+
+      attr_reader :request
+
+      def params
+        @request.query_parameters
+      end
+
+      def header(header, value)
+        @response.headers[header] = value
+      end
+    end
+  end
+end