Swapped grunt-cssnano for grunt-postcss+cssnano

- The grunt-cssnano plugin is old and no longer maintained
- It uses insecure dependencies that don't really impact us, but we want to get rid of warnings
- Swapping for grunt-postcss+cssnano is a more up-to-date way of sorting this out

Gruntfile.js

@@ -38,6 +38,7 @@ const configureGrunt = function (grunt) {
     //
     // Find all of the task which start with `grunt-` and load them, rather than explicitly declaring them all
     require('matchdep').filterDev(['grunt-*', '!grunt-cli']).forEach(grunt.loadNpmTasks);
+    grunt.loadNpmTasks('@lodder/grunt-postcss');
 
     var cfg = {
         // #### Common paths used by tasks
@@ -276,10 +277,12 @@ const configureGrunt = function (grunt) {
             }
         },
 
-        cssnano: {
+        postcss: {
             prod: {
                 options: {
-                    sourcemap: false
+                    processors: [
+                        require('cssnano')() // minify the result
+                    ]
                 },
                 files: {
                     'core/server/public/ghost.min.css': 'core/server/public/ghost.css'
@@ -542,7 +545,7 @@ const configureGrunt = function (grunt) {
     //
     // It is otherwise the same as running `grunt`, but is only used when running Ghost in the `production` env.
     grunt.registerTask('prod', 'Build JS & templates for production',
-        ['subgrunt:prod', 'uglify:prod', 'cssnano:prod', 'master-warn']);
+        ['subgrunt:prod', 'uglify:prod', 'postcss:prod', 'master-warn']);
 
     // ### Live reload
     // `grunt dev` - build assets on the fly whilst developing

package.json

@@ -134,6 +134,8 @@
     "sqlite3": "4.1.1"
   },
   "devDependencies": {
+    "@lodder/grunt-postcss": "^2.0.1",
+    "cssnano": "^4.1.10",
     "eslint": "6.8.0",
     "eslint-plugin-ghost": "1.0.1",
     "grunt": "1.1.0",
@@ -145,7 +147,6 @@
     "grunt-contrib-symlink": "1.0.0",
     "grunt-contrib-uglify": "4.0.1",
     "grunt-contrib-watch": "1.1.0",
-    "grunt-cssnano": "3.0.0",
     "grunt-express-server": "0.5.4",
     "grunt-mocha-cli": "6.0.0",
     "grunt-shell": "3.0.1",