[notifications] Email template and dispatch integration — `feed.url_updated`

[davidgamez]

Description

Create the email content and dispatcher integration for feed.url_updated.

Include the affected feed, previous and new URL or replacement feed, and a link to the feed page.

The template need to support digest mode only for MVP. In digest multiple notifications from the same type are included in the message. (Not MVP) in no-digest mode a single notification per notification type is sent.