[Config] Acknowledge when Advanced Settings response

This adds a return value to the `Promise` returned by `config.set`, which
allows callers to `await` it  and verify that their setting was applied
without checking the settings (which is a race condition).

src/ui/public/config/__tests__/config.js

@@ -57,6 +57,13 @@ describe('config component', function () {
       expect(config.set).withArgs('unrecognizedProperty', 'somevalue').to.not.throwException();
       expect(config.get('unrecognizedProperty')).to.be('somevalue');
     });
+
+    // there is currently no way to make it return false without significant changes to the code
+    it('returns true for success', async () => {
+      expect(await config.set('random', 'value')).to.be(true);
+      // setting to the same should set it to true as well
+      expect(await config.set('random', 'value')).to.be(true);
+    });
   });
 
   describe('#$bind', function () {

src/ui/public/config/config.js

@@ -63,18 +63,20 @@ any custom setting configuration watchers for "${key}" may fix this issue.`);
     const newVal = key in defaults && defaults[key].defaultValue === value ? null : value;
     const unchanged = oldVal === newVal;
     if (unchanged) {
-      return Promise.resolve();
+      return Promise.resolve(true);
     }
     const initialVal = declared ? config.get(key) : undefined;
     localUpdate(key, newVal, initialVal);
 
     return delayedUpdate(key, newVal)
       .then(updatedSettings => {
         settings = mergeSettings(defaults, updatedSettings);
+        return true;
       })
       .catch(reason => {
         localUpdate(key, initialVal, config.get(key));
         notify.error(reason);
+        return false;
       });
   }