[ENHANCEMENT] Add support for web feature subscriptions #1779
Description
It would be nice for dashboard users to be able to subscribe to changes to their bookmarks. For example, if a user bookmarks a list of features including some that are Limited availability, they would probably want to be notified when those features eventually become Baseline Newly available, and probably also when it progresses to Widely available.
In terms of how to notify users, email would be the most straightforward delivery mechanism. Logged in users are already authenticated with GitHub, which provides their email address. The dashboard could also expose an RSS feed of the latest changes in Baseline status for a given bookmark, enabling users to consume the data programmatically. It could potentially also support something like web hooks, where the server notifies another server of the user's choosing, for even more integration points with apps like Slack or GitHub.