make mailbox domain nullable

for apps that do not use sendmail/recvmail addon, these are now null.
otherwise, there is no way to edit the mailbox in the UI

part of #669

src/addons.js

@@ -524,7 +524,14 @@ function teardownAddons(app, addons, callback) {
         debugApp(app, 'Tearing down addon %s with options %j', addon, addons[addon]);
 
         KNOWN_ADDONS[addon].teardown(app, addons[addon], iteratorCallback);
-    }, callback);
+    }, function (error) {
+        if (error) return callback(error);
+
+        if (app.manifest.addons.sendmail || app.manifest.addons.recvmail) return callback();
+
+        // clear mailbox name if neither are used
+        appdb.update(app.id, { mailboxName: null, mailboxDomain: null }, callback);
+    });
 }
 
 function backupAddons(app, addons, callback) {