Bump Sprockets to 4.2.1

* Replace sprockets with sprockets-rails

Gemfile

@@ -37,7 +37,7 @@ gem 'sassc-rails'
 gem 'scenic'
 gem 'sentry-rails'
 gem 'sidekiq', '6.5.11'
-gem 'sprockets', '< 4'
+gem 'sprockets-rails'
 gem 'state_machines-activerecord'
 gem 'string_scrubber'
 gem 'turnout'

Gemfile.lock

@@ -465,9 +465,9 @@ GEM
     spring (4.1.1)
     spring-commands-rspec (1.0.4)
       spring (>= 0.9.1)
-    sprockets (3.7.2)
+    sprockets (4.2.1)
       concurrent-ruby (~> 1.0)
-      rack (> 1, < 3)
+      rack (>= 2.2.4, < 4)
     sprockets-rails (3.4.2)
       actionpack (>= 5.2)
       activesupport (>= 5.2)
@@ -590,7 +590,7 @@ DEPENDENCIES
   sidekiq (= 6.5.11)
   simplecov
   spring-commands-rspec
-  sprockets (< 4)
+  sprockets-rails
   state_machines-activerecord
   string_scrubber
   timecop

app/assets/config/manifest.js

@@ -0,0 +1,15 @@
+//= link_tree ../images
+//= link application.js
+//= link application.css
+//= link application-ie6.css
+//= link application-ie7.css
+//= link application-ie8.css
+//= link govuk-template.css
+//= link govuk-template-ie6.css
+//= link govuk-template-ie7.css
+//= link govuk-template-ie8.css
+//= link govuk-template-print.css
+//= link fonts.css
+//= link email.css
+//= link metrics.js
+//= link jasmine-jquery.js

config/initializers/assets.rb

@@ -11,22 +11,4 @@
 # Precompile additional assets.
 # application.js, application.css, and all non-JS/CSS in the app/assets
 # folder are already added.
-Rails.application.config.assets.precompile += %w[
-  application.css
-  application-ie6.css
-  application-ie7.css
-  application-ie8.css
-  email.css
-  *.png
-  *.svg
-  *.jpg
-  favicon.ico
-  metrics.css
-  metrics.js
-  gov.uk_logotype_crown.svg
-  *.eot
-  *.svg
-  *.ttf
-  *.woff
-  jasmine-jquery.js
-]
+# Rails.application.config.assets.precompile += %w( admin.js admin.css )