Switch to using file locations in app-config.yaml

Having file locations makes the local workflow of editing entities, e.g. for testing much easier. When you add new files, you don't run into a circular dependency anymore, previously you couldn't test whether the configuration worked before merging it into master. It also resolves the hard dependency on the GitHub repository - which allows to do "breaking changes" to the files without having to worry that existing instances used for development locally will break. However after this change we should have a grace period in that we should still be careful.

However there might be hidden risks that might break others workflows.

We have this configuration locally for a long time and it works well.

app-config.yaml

@@ -185,33 +185,35 @@ catalog:
       #    groupFilter: securityEnabled eq false and mailEnabled eq true and groupTypes/any(c:c+eq+'Unified')
 
   locations:
+    # File locations are relative to the current working directory of the
+    # backend, for example packages/backend/.
     # Backstage example components
-    - type: url
-      target: https://github.com/backstage/backstage/blob/master/packages/catalog-model/examples/all-components.yaml
+    - type: file
+      target: ../catalog-model/examples/all-components.yaml
     # Example component for github-actions
-    - type: url
-      target: https://github.com/backstage/backstage/blob/master/plugins/github-actions/examples/sample.yaml
+    - type: file
+      target: ../../plugins/github-actions/examples/sample.yaml
     # Example component for TechDocs
-    - type: url
-      target: https://github.com/backstage/backstage/blob/master/plugins/techdocs-backend/examples/documented-component/catalog-info.yaml
+    - type: file
+      target: ../../plugins/techdocs-backend/examples/documented-component/catalog-info.yaml
     # Backstage example APIs
-    - type: url
-      target: https://github.com/backstage/backstage/blob/master/packages/catalog-model/examples/all-apis.yaml
+    - type: file
+      target: ../catalog-model/examples/all-apis.yaml
     # Backstage example resources
-    - type: url
-      target: https://github.com/backstage/backstage/blob/master/packages/catalog-model/examples/all-resources.yaml
+    - type: file
+      target: ../catalog-model/examples/all-resources.yaml
     # Backstage example systems
-    - type: url
-      target: https://github.com/backstage/backstage/blob/master/packages/catalog-model/examples/all-systems.yaml
+    - type: file
+      target: ../catalog-model/examples/all-systems.yaml
     # Backstage example domains
-    - type: url
-      target: https://github.com/backstage/backstage/blob/master/packages/catalog-model/examples/all-domains.yaml
+    - type: file
+      target: ../catalog-model/examples/all-domains.yaml
     # Backstage example templates
-    - type: url
-      target: https://github.com/backstage/backstage/blob/master/plugins/scaffolder-backend/sample-templates/all-templates.yaml
+    - type: file
+      target: ../../plugins/scaffolder-backend/sample-templates/all-templates.yaml
     # Backstage example groups and users
-    - type: url
-      target: https://github.com/backstage/backstage/blob/master/packages/catalog-model/examples/acme-corp.yaml
+    - type: file
+      target: ../catalog-model/examples/acme-corp.yaml
 
 scaffolder:
   github: