Enabled babel sourcemaps in admin build (#18857)

no issue

- Somewhere along the process of building the admin app, the sourcemaps
are getting corrupted
- This commit is to test the theory that babel is the source of the
corruption, because it isn't generating sourcemaps, so we are missing a
step in the process

ghost/admin/ember-cli-build.js

@@ -83,7 +83,8 @@ module.exports = function (defaults) {
         babel: {
             plugins: [
                 require.resolve('babel-plugin-transform-react-jsx')
-            ]
+            ],
+            sourceMaps: true
         },
         'ember-cli-babel': {
             optional: ['es6.spec.symbols'],