Use Preferences abstraction instead of manual localStorage interaction

Default values and localStorage manipulation should be implemented in the
Prefereces class.

src/peertube-api.ts

@@ -16,9 +16,10 @@
 
 import * as _ from 'lodash/fp';
 import constants from './constants';
+import Preferences from './preferences';
 
 async function fetchAPI(path: string, query?: Object) {
-    const instance = _.getOr(constants.peertubeAPI.defaultInstance, 'searchInstance', await browser.storage.local.get()).toString();
+    const instance = (await Preferences.getPreferences()).searchInstance;
 
     let url = `https://${instance}/${constants.peertubeAPI.endpoint}/${path}`;