[notifications] Email template and dispatch integration — `location.feed_added`

[davidgamez]

Description

Create the email content and dispatcher integration for location.feed_added.

Include the location, newly added feed, provider or feed name when available, 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.