Feature: Persistent user settings for custom modes

[alvinsw]

Allow user to add or remove the source of a mode either remote (from github etc) or local and save the information to browser's local storage.
Two ways to implement this in the UI:

1. Add a setting button to the right of the mode select box. Clicking the button will bring up a dialog to manage the mode sources
2. Add remove button to each entry of the select box, and two add buttons for adding remote and local sources