PERF: reuse renderer when rendering email templates

Previous to this fix we were leaking methods on the internal action view
template class per render.

This caused email generation to be very low and a steady memory leak in the
application in sidekiq when sending out emails

The behavior change is new to Rails 6 so this fix does not need to be
backported into stable.

app/mailers/user_notifications.rb

@@ -589,7 +589,7 @@ class UserNotifications < ActionMailer::Base
       end
 
       unless translation_override_exists
-        html = UserNotificationRenderer.with_view_paths(Rails.configuration.paths["app/views"]).render(
+        html = UserNotificationRenderer.instance.render(
           template: 'email/notification',
           format: :html,
           locals: { context_posts: context_posts,

app/services/user_notification_renderer.rb

@@ -4,4 +4,10 @@ class UserNotificationRenderer < ActionView::Base
   include ApplicationHelper
   include UserNotificationsHelper
   include EmailHelper
+
+  def self.instance
+    @instance ||= UserNotificationRenderer.with_view_paths(
+      Rails.configuration.paths["app/views"]
+    )
+  end
 end

lib/email/message_builder.rb

@@ -94,9 +94,7 @@ module Email
         html_override.gsub!("%{respond_instructions}", "")
       end
 
-      html = UserNotificationRenderer.with_view_paths(
-        Rails.configuration.paths["app/views"]
-      ).render(
+      html = UserNotificationRenderer.instance.render(
         template: 'layouts/email_template',
         format: :html,
         locals: { html_body: html_override.html_safe }

lib/email/renderer.rb

@@ -18,9 +18,7 @@ module Email
       style = if @message.html_part
         Email::Styles.new(@message.html_part.body.to_s, @opts)
       else
-        unstyled = UserNotificationRenderer.with_view_paths(
-          Rails.configuration.paths["app/views"]
-        ).render(
+        unstyled = UserNotificationRenderer.instance.render(
           template: 'layouts/email_template',
           format: :html,
           locals: { html_body: PrettyText.cook(text).html_safe }