Allow users to subscribe to activity page feeds

## Feature Request Form

Users may wish to subscribe to Atom/RSS feeds that correspond to a particular search on activity pages, in order to keep up with activity on a particular page, tag, or keyword.

We have existing Atom/RSS feeds at:

`/stream.rss`
`/stream.atom`

but they use an old and inflexible syntax for expressing searches that isn't the syntax we use for activity pages. Moreover, these feeds are not linked from activity pages using the appropriate `` tags as they are from the `/stream` page.

### Proposed solution

- Create new RSS/Atom feeds at (say) `/search.rss` and `/search.atom` which use the same query syntax as activity pages.
- Embed appropriate link tags into the HTML activity pages to allow users to subscribe to a specific activity pages view from their browser.

### Additional context

- https://github.com/hypothesis/h/issues/3003