Added EmailsDownload controller and routes

Removed original method to return emails file from Newsletters controller and NewsletterZip class, included `rubyzip` gem that's no longer necessary.

lib/newsletter_zip.rb

@@ -1,25 +0,0 @@
-require 'zip'
-class NewsletterZip
-  attr_accessor :filename
-
-  def initialize(filename)
-    @filename = filename
-  end
-
-  def emails
-    User.newsletter.pluck(:email).join("\n")
-  end
-
-  def path
-    Rails.root + "/tmp/#{filename}.zip"
-  end
-
-  def create
-    Zip::File.open(path, Zip::File::CREATE) do |zipfile|
-      zipfile.get_output_stream("#{filename}.txt") do |file|
-        file.write emails
-      end
-    end
-  end
-
-end