Generate asset to test CDN compression config

Cloudfront, our CDN, sometimes decides not to gzip
assets. Because of this, we're going to gzip them
ourselves prior to upload instead.

This will involve:
1. adding gzipping to the make task that uploads
   them
2. turning compression off in Cloudfront

There is already a pull request up for number 1:

https://github.com/alphagov/notifications-admin/pull/3733

Because deploying all this will, at some point,
create a state where Cloudfront is set to compress
assets that are already compressed, we need to
test that it doesn't re-compress them.

This adds a frontend build task that generates a
test asset which is:
- a copy of app/static/stylesheets/main.css
- renamed to include a MD5 SHA of its contents
- already gzipped

Once deployed, the test will be to:
1. download the asset from the live environment
2. unzip it
3. diff it against app/static/stylesheets/main.css

gulpfile.js

@@ -18,7 +18,9 @@ plugins.base64 = require('gulp-base64-inline');
 plugins.cleanCSS = require('gulp-clean-css');
 plugins.concat = require('gulp-concat');
 plugins.cssUrlAdjuster = require('gulp-css-url-adjuster');
+plugins.gzip = require('gulp-gzip');
 plugins.jshint = require('gulp-jshint');
+plugins.md5 = require('gulp-md5');
 plugins.prettyerror = require('gulp-prettyerror');
 plugins.rollup = require('gulp-better-rollup')
 plugins.sass = require('gulp-sass');
@@ -108,6 +110,14 @@ const copy = {
       return src(paths.npm + 'leaflet/dist/leaflet.js')
         .pipe(dest(paths.dist + 'javascripts/'))
     }
+  },
+  tests: {
+    css: () => { 
+      return src(paths.dist + 'stylesheets/main.css')
+        .pipe(plugins.md5({ separator: '-' }))
+        .pipe(plugins.gzip({ gzipOptions: { level: 9 }, preExtension: 'gz' }))
+        .pipe(dest(paths.dist + 'test/'))
+    }
   }
 };
 
@@ -290,7 +300,8 @@ const defaultTask = parallel(
     series(
       javascripts
     ),
-    sass
+    sass,
+    copy.tests.css
   )
 );
 

package.json

@@ -47,7 +47,9 @@
   },
   "devDependencies": {
     "gulp-css-url-adjuster": "0.2.3",
+    "gulp-gzip": "1.4.2",
     "gulp-jshint": "2.1.0",
+    "gulp-md5": "0.1.3",
     "gulp-prettyerror": "1.2.1",
     "gulp-sass-lint": "1.4.0",
     "jest": "24.7.1",