Prevent memory bloat in ActiveJob children

`ActiveJob` attaches a single `LogSubscriber` to be notified of events.
When one job enqueues another, that's treated as a child event and
stored on the same stack as the parent. If a large number of children
are enqueued that leads to R14 errors on Heroku.

The fix here is to unsubscribe all existing "enqueue" listeners and
then add a new subscriber for just that event. The original handler
is copied from the parent class using `define_method` so that it's
picked up by `attach_to`.

Related:

rails/rails#21036
rails/rails#5932

Before and after:

![Heroku metrics chart showing 1GB drop in memory usage after this change deployed](https://user-images.githubusercontent.com/47136/28436362-b51b3d1e-6d64-11e7-8b24-fc67927e7081.png)

lib/suspenders.rb

@@ -1,6 +1,7 @@
 require 'suspenders/version'
 require 'suspenders/generators/app_generator'
 require "suspenders/generators/enforce_ssl_generator"
+require "suspenders/generators/initialize_active_job_generator"
 require 'suspenders/generators/static_generator'
 require 'suspenders/generators/stylesheet_base_generator'
 require 'suspenders/actions'

lib/suspenders/generators/app_generator.rb

@@ -217,6 +217,7 @@ def remove_routes_comment_lines
 
     def generate_default
       run("spring stop")
+      generate("suspenders:initialize_active_job")
       generate("suspenders:enforce_ssl")
       generate("suspenders:static")
       generate("suspenders:stylesheet_base")

lib/suspenders/generators/initialize_active_job_generator.rb

@@ -0,0 +1,19 @@
+require "rails/generators"
+
+module Suspenders
+  class InitializeActiveJobGenerator < Rails::Generators::Base
+    source_root(
+      File.expand_path(
+        File.join("..", "..", "..", "templates"),
+        File.dirname(__FILE__),
+      ),
+    )
+
+    def initialize_active_job
+      copy_file(
+        "active_job.rb",
+        "config/initializers/active_job.rb",
+      )
+    end
+  end
+end

spec/features/new_project_spec.rb

@@ -100,6 +100,11 @@
     expect(env).to include "RACK_MINI_PROFILER=0"
   end
 
+  it "initializes ActiveJob to avoid memory bloat" do
+    expect(File).
+      to exist("#{project_path}/config/initializers/active_job.rb")
+  end
+
   it "creates a rack-mini-profiler initializer" do
     expect(File).
       to exist("#{project_path}/config/initializers/rack_mini_profiler.rb")

templates/active_job.rb

@@ -0,0 +1,13 @@
+require "active_job/logging"
+
+ActiveSupport::Notifications.unsubscribe("enqueue.active_job")
+
+module ActiveJob
+  module Logging
+    class EnqueueLogSubscriber < LogSubscriber
+      define_method :enqueue, instance_method(:enqueue)
+    end
+  end
+end
+
+ActiveJob::Logging::EnqueueLogSubscriber.attach_to(:active_job)