Run yarn linting as part of default rake task

This also updates the Rakefile to be more consistent with the other ones
on GOV.UK apps, by clearing a default task if it exists and re-defining
default as all lint and test tasks.

Rakefile

@@ -5,12 +5,5 @@ require File.expand_path("config/application", __dir__)
 
 Rails.application.load_tasks
 
-unless Rails.env.production?
-  require "rubocop/rake_task"
-
-  RuboCop::RakeTask.new(:rubocop) do |t|
-    t.patterns = %w[app config test Gemfile]
-  end
-
-  task default: %i[spec rubocop]
-end
+Rake::Task[:default].clear if Rake::Task.task_defined?(:default)
+task default: %i[lint test jasmine:ci]

lib/tasks/lint.rake

@@ -0,0 +1,5 @@
+desc "Run all linters"
+task lint: :environment do
+  sh "bundle exec rubocop"
+  sh "yarn run lint"
+end