Identify links by an ID

[alinnert]

Currently links that are hidden are identified by its URL. That means if a URL changes the link gets visible again because only the previous URL is stored in the localStorage.

This is why it's better to use an ID to identify a link instead.

Todo:

• Update the JSON schema
• Add IDs to all current URLs
• Write a migration that changes all URLs in the local storage to IDs, so users don't lose their settings