Added sourcesContent key to admin sourcemaps (#18855)

no issue

- The sourcemaps currently generated by the admin build do not include
the `sourcesContent` key
- This commit is to experiment with this change, in particular to see if
it improves the stacktraces available in Sentry from errors in the admin
app
- Currently the stacktraces in admin show the minified code — hopefully
this change will make the stacktraces show the original source code to
make it easier to debug issues from within Sentry
- Might thoroughly break the sourcemaps, in which case I will revert this

ghost/admin/ember-cli-build.js

@@ -205,6 +205,7 @@ module.exports = function (defaults) {
         autoImport: {
             publicAssetURL,
             webpack: {
+                devtool: 'source-map',
                 resolve: {
                     fallback: {
                         util: require.resolve('util'),