replace usage of _.extend with Object.assign

src/settings.js

@@ -471,7 +471,7 @@ async function setBackupCredentials(credentials) {
     // preserve these fields
     const extra = _.pick(currentConfig, 'retentionPolicy', 'schedulePattern', 'copyConcurrency', 'syncConcurrency', 'memoryLimit', 'downloadConcurrency', 'deleteConcurrency', 'uploadPartSize');
 
-    const backupConfig = _.extend({}, credentials, extra);
+    const backupConfig = Object.assign({}, credentials, extra);
 
     backups.cleanupCacheFilesSync();
 
@@ -734,7 +734,7 @@ async function setAppstoreWebToken(token) {
 async function list() {
     const settings = await database.query(`SELECT ${SETTINGS_FIELDS} FROM settings WHERE value IS NOT NULL ORDER BY name`);
 
-    const result = _.extend({ }, gDefaults);
+    const result = Object.assign({}, gDefaults);
     settings.forEach(function (setting) { result[setting.name] = setting.value; });
 
     // convert booleans