Extract AWS_S3_BUCKET_NAME env var into app config

So it's easier to stub.

Unfortunately I've had to disable a Rubocop rule around one line. This
violation is due to the digit `3` in the variable name. It looks as if
the constraints for this rule have been relaxed [1] in Rubocop v0.44.0,
but we're currently using v0.43.0 and upgrading means first upgrading
the constraints on the Rubocop version in the specification for the
`govuk-lint` gem and then upgrading that gem in this project. I'm
going to do that separately and I'll make a note to remove this
disabling of the rule when I do that.

[1]: rubocop/rubocop@833aad6

config/initializers/aws.rb

@@ -1,3 +1,5 @@
+AssetManager::Application.config.aws_s3_bucket_name = ENV['AWS_S3_BUCKET_NAME']
+
 Aws.config.update(
   logger: Rails.logger
 )

lib/services.rb

@@ -2,6 +2,9 @@
 
 module Services
   def self.cloud_storage
-    @cloud_storage ||= S3Storage.build(ENV['AWS_S3_BUCKET_NAME'])
+    # rubocop:disable Style/VariableNumber
+    aws_s3_bucket_name = AssetManager::Application.config.aws_s3_bucket_name
+    # rubocop:enable Style/VariableNumber
+    @cloud_storage ||= S3Storage.build(aws_s3_bucket_name)
   end
 end