Move Content Security Policy config to conventional file

This resolves this app having both a csp and content_security_policy.rb
file.

We choose the latter one as it is a Rails default that will get
automatically added on Rails upgrades.

config/initializers/content_security_policy.rb

@@ -1,26 +1 @@
-# Be sure to restart your server when you modify this file.
-
-# Define an application-wide content security policy
-# For further information see the following documentation
-# https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy
-
-# Rails.application.configure do
-#   config.content_security_policy do |policy|
-#     policy.default_src :self, :https
-#     policy.font_src    :self, :https, :data
-#     policy.img_src     :self, :https, :data
-#     policy.object_src  :none
-#     policy.script_src  :self, :https
-#     policy.style_src   :self, :https
-#     # Specify URI for violation reports
-#     # policy.report_uri "/csp-violation-report-endpoint"
-#   end
-#
-#   # Generate session nonces for permitted importmap and inline scripts
-#   config.content_security_policy_nonce_generator = ->(request) { request.session.id.to_s }
-#   config.content_security_policy_nonce_directives = %w(script-src)
-#
-#   # Report CSP violations to a specified URI. See:
-#   # https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy-Report-Only
-#   # config.content_security_policy_report_only = true
-# end
+GovukContentSecurityPolicy.configure