replace subfolder on cdn url conversion between general cdn and s3 (#7764)

When both a cdn URL and an s3 cdn URL defined, subfolder paths were leaking
through to the s3 cdn URL. If we are replacing the cdn url with the s3_cdn url,
we also need to make sure that the subpath is removed as well, as it appears in
the original cdn url.

The test should give a fairly good gist of the situations - in subfolder
situations where s3_cdn and a cdn is defined:
`asset_path` returns the asset with a subfolder, in the form `{cdn_url}/{subfolder}/{asset_path}`

Currently this is being replaced to `{s3_cdn_url}/{subfolder}/{asset_path}`
I am proposing we change this to: `{s3_cdn_url}/{asset_path}` as it seems like
for s3_cdn urls we should not be carrying around app subfolder pathing anywhere
we are looking up s3 paths.

app/helpers/application_helper.rb

@@ -1,3 +1,4 @@
+# coding: utf-8
 # frozen_string_literal: true
 require 'current_user'
 require 'canonical_url'
@@ -62,7 +63,8 @@ module ApplicationHelper
 
     if GlobalSetting.use_s3? && GlobalSetting.s3_cdn_url
       if GlobalSetting.cdn_url
-        path = path.gsub(GlobalSetting.cdn_url, GlobalSetting.s3_cdn_url)
+        folder = ActionController::Base.config.relative_url_root || "/"
+        path = path.gsub(File.join(GlobalSetting.cdn_url, folder, "/"), File.join(GlobalSetting.s3_cdn_url, "/"))
       else
         # we must remove the subfolder path here, assets are uploaded to s3
         # without it getting involved

spec/helpers/application_helper_spec.rb

@@ -1,3 +1,4 @@
+# coding: utf-8
 # frozen_string_literal: true
 
 require 'rails_helper'
@@ -32,6 +33,13 @@ describe ApplicationHelper do
         expect(helper.preload_script("application")).to include('https://s3cdn.com/assets/application.js')
       end
 
+      it "replaces cdn URLs with s3 cdn subfolder paths" do
+        global_setting :s3_cdn_url, 'https://s3cdn.com/s3_subpath'
+        set_cdn_url "https://awesome.com"
+        ActionController::Base.config.relative_url_root = "/community"
+        expect(helper.preload_script("application")).to include('https://s3cdn.com/s3_subpath/assets/application.js')
+      end
+
       it "returns magic brotli mangling for brotli requests" do
 
         helper.request.env["HTTP_ACCEPT_ENCODING"] = 'br'