Check subscription status

Sometimes I forget that I have already subscribed to a RSS feed, and only realize it when I click into it.

Start by tracing the browser extension's subscription flow from selecting an RSS feed through the point where a user can subscribe. Determine where the current subscription state can be checked and how that state is presented before the user clicks; done means an existing subscription is apparent without requiring the user to open the feed.

Written by the indexing model from the issue text.