Disable ActiveStorage routes

We aren't using ActiveStorage, but Rails was including its routes
anyway.

In Rails 6.1 there will be an option to disable these routes [1], but
for now we're changing the line requiring "rails/all" to the values
generated by a new Rails application with the --skip-active-storage
flag.

[1] https://github.com/rails/rails/commit/3cf65bcb8

config/application.rb

@@ -1,6 +1,17 @@
 require_relative "boot"
 
-require "rails/all"
+require "rails"
+# Pick the frameworks you want:
+require "active_model/railtie"
+require "active_job/railtie"
+require "active_record/railtie"
+# require "active_storage/engine"
+require "action_controller/railtie"
+require "action_mailer/railtie"
+require "action_view/railtie"
+require "action_cable/engine"
+require "sprockets/railtie"
+require "rails/test_unit/railtie"
 
 # Require the gems listed in Gemfile, including any gems
 # you've limited to :test, :development, or :production.