dev: add services property to extensions

A small change that adds ".services" to the "extension" global in an
extension, so extensions can more easily talk to other services.

src/backend/src/Extension.js

@@ -61,6 +61,17 @@ class Extension extends AdvancedBase {
         return db;
     }
 
+    get services () {
+        const services = this.service.values.get('services');
+        if ( ! services ) {
+            throw new Error(
+                'extension tried to access "services" before it was ' +
+                'initialized'
+            );
+        }
+        return services;
+    }
+
     /**
      * This will create a GET endpoint on the default service.
      * @param {*} path - route for the endpoint

src/backend/src/ExtensionService.js

@@ -80,6 +80,8 @@ class ExtensionService extends BaseService {
     }
     async _init (args) {
         this.state = args.state;
+        
+        this.state.values.set('services', this.services);
 
         // Create database access object for extension
         const db = this.services.get('database').get(DB_WRITE, 'extension');