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.

app/controllers/admin/emails_download_controller.rb

@@ -0,0 +1,18 @@
+class Admin::EmailsDownloadController < Admin::BaseController
+  def index
+  end
+
+  def generate_csv
+    users_segment = params[:users_segment]
+    filename = t("admin.segment_recipient.#{users_segment}")
+
+    csv_file = users_segment_emails_csv(users_segment)
+    send_data csv_file, filename: "#{filename}.csv"
+  end
+
+  private
+
+  def users_segment_emails_csv(users_segment)
+    UserSegments.send(users_segment).pluck(:email).to_csv
+  end
+end