Fixed Koenig SVG assets after ember-svg-jar switch

no issue
- https://github.com/TryGhost/Ghost-Admin/pull/970 worked for the general public icons but failed for koenig icons because of the incorrect config parameter name (`paths` -> `sourceDirs`)
- disabled the `stripPath` config so that there's no conflicts between public (`icon`) and koenig (`koenig/icon`) icons

ghost/admin/ember-cli-build.js

@@ -159,7 +159,9 @@ module.exports = function (defaults) {
             }
         },
         svgJar: {
-            paths: [
+            strategy: 'inline',
+            stripPath: false,
+            sourceDirs: [
                 'public/assets/icons',
                 'lib/koenig-editor/public/icons'
             ],