[ACA-1619] automatically show content metadata (#1104)

* [ACA-1619] overwrite the app.config metadata settings only of the 'content-metadata-presets' is non-empty

* [ACA-1619] app config to show automatically all metadata aspects

- not extendable version

* [ACA-1619] app extension to show automatically all metadata aspects

- extendable version
- the app.config setting is overwritten by the one with ids from app.extensions
- could remove completely the content metadata setting from app.json

* [ACA-1619] allow 'disabled' property on content metadata configuration

* [ACA-1619] test content-metadata plugin extension

 - allow 'disabled' property on content metadata configuration

* [ACA-1619] remove test reference to metadata plugin

* [ACA-1619] backwards compatibility for metadata plugin extensions

- add back initial structure (& ids) of content-metadata "custom" preset
- revert change to sample metadata extension plugin

* [ACA-1619] forgotten change

* [ACA-1619] tests

src/app/extensions/extension.service.ts

@@ -307,6 +307,10 @@ export class AppExtensionService implements RuleContext {
       config,
       'features.content-metadata-presets'
     );
+    if (!elements.length) {
+      return null;
+    }
+
     let presets = {};
     presets = this.filterDisabled(mergeObjects(presets, ...elements));