Add extension to middleman-sprockets to get http_prefix in Sass

We want to be able to prefix directory paths with the http_prefix in
Sass, however normally the `asset-path` helper tries to add a file
extension, so this commit adds a small extension that skips the usual
logic if the path ends with `/`.

The extension works by reaching into the `Sprockets::Envionment` instance
in the `middleman-sprockets` extension [1], and replacing the definition
of the method `asset_path` [2]. Sprockets turns any calls to
`asset-path` in any CSS/Sass it processes into a call to this method, so
this is our way to smuggle information into our Sass.

[1]: https://github.com/middleman/middleman-sprockets/blob/a32cbe3c8ca129b608bdc852c7ff7ad489f9a087/lib/middleman-sprockets/extension.rb#L28
[2]: https://github.com/middleman/middleman-sprockets/blob/a32cbe3c8ca129b608bdc852c7ff7ad489f9a087/lib/middleman-sprockets/extension.rb#L58

lib/govuk_tech_docs.rb

@@ -22,6 +22,7 @@
 require "govuk_tech_docs/unique_identifier_generator"
 require "govuk_tech_docs/warning_text_extension"
 require "govuk_tech_docs/api_reference/api_reference_extension"
+require "govuk_tech_docs/http_prefix_extension"
 
 module GovukTechDocs
   # Configure the tech docs template
@@ -65,6 +66,7 @@ def self.configure(context, options = {})
     context.activate :unique_identifier
     context.activate :warning_text
     context.activate :api_reference
+    context.activate :sass_http_prefix
 
     context.helpers do
       include GovukTechDocs::TableOfContents::Helpers

lib/govuk_tech_docs/http_prefix_extension.rb

@@ -0,0 +1,21 @@
+module GovukTechDocs
+  class SassHttpPrefixExtension < Middleman::Extension
+    def after_configuration
+      app.extensions[:sprockets].environment.context_class.class_eval do
+        # Override the `asset-path()` helper available in Sprockets to
+        # return a directory rather than a file if the path ends with `/`
+        alias_method :orig_asset_path, :asset_path
+
+        def asset_path path, options = {}
+          if options.empty? && path.end_with?("/")
+            File.join(*[app.config[:http_prefix], path].compact)
+          else
+            orig_asset_path path, options
+          end
+        end
+      end
+    end
+  end
+end
+
+::Middleman::Extensions.register(:sass_http_prefix, GovukTechDocs::SassHttpPrefixExtension)