RSS/Atom Feed channel title & link should name & link to the search behind the feed

Currently, the RSS generated from:

```https://hypothes.is/stream.rss?tags=sci:3004```

contains

```

Hypothesis Stream
https://hypothes.is/stream

The Web. Annotated
Sun, 16 Sep 2018 21:22:31 -0000
```
but profitably could be returned as below (where I have suggested edits for the title, link, atom:link and description):

```

Hypothesis Annotations
https://hypothes.is/search?q=tag%3Asci%3A3004

Query: tags=sci:3004
Sun, 16 Sep 2018 21:22:31 -0000
```

Doing so would be more informative as to the content of the stream when presented in an RSS context, and provide opportunity to see the full stream contents in the hypothesis environment, further engaging the user.

Not doing so brings the user into a stream displaying content that is different from that of the RSS feed, which is highly disorienting.

Start from the stream.rss endpoint shown in the issue and inspect how its channel title, link, Atom self-link, and description are generated. Compare the output for the tagged query with the proposed values; done means the feed identifies and links to the search behind it without changing the feed contents.

Written by the indexing model from the issue text.