Correct bouncer links as they appear in RSS streams

**Describe the bug**
Per https://hypothes-is.slack.com/archives/C2BLQDKHA/p1588956474352200:

> In a feed like this:
> https://hypothes.is/stream.atom?uri=http://www.theatlantic.com/politics/archive/2015/07/tanehisi-coates-between-the-world-and-me/397619/
>
> The links are of the form:
> https://hypothes.is/a/Xz3mnBQiEem3MeOgV9_6SQ
>
> They probably should be like this:
> https://hyp.is/Xz3mnBQiEem3MeOgV9_6SQ
>
>Why this is the case: the RSS views predated bouncer.

**To Reproduce**
See https://hypothes.is/stream.atom?uri=http://www.theatlantic.com/politics/archive/2015/07/tanehisi-coates-between-the-world-and-me/397619/

Click on https://hypothes.is/a/Xz3mnBQiEem3MeOgV9_6SQ

**Expected behavior**
We would expect the links in an RSS stream to work like this link does instead: https://hyp.is/Xz3mnBQiEem3MeOgV9_6SQ

**Additional context**
https://hypothesis.zendesk.com/agent/tickets/9964

Start with the stream.atom URL in the reproduction steps and inspect the RSS views mentioned in the report. Compare the generated annotation links with the expected hyp.is link and verify the RSS output uses the bouncer URL format.

Written by the indexing model from the issue text.