prevent ongoing document equivalence explosions

The following URL was assigned in a course:

https://www.npr.org/sections/health-shots/2010/08/11/129127432/paul-longmore-historian-and-advocate-for-disabled-dies

The course dashboard displayed this list of annotated documents:


These are all the "same" document, with document_id 22734. Most users won't (yet) be seeing this dashboard view, but for everyone our client will load and try to anchor ~2500 annotations, the vast majority of which do not apply to this article.

This happens because:

1. The docs share this common metadata declaration: {href: "https://feeds.npr.org/feeds/103537970/feed.json", rel: "alternate", type: "application/json"}

2. We formerly used `rel="alternate"` to form equivalences, until we realized that this pattern -- a feed URL that's common to a whole class of documents on a site -- was causing an explosion of unwanted equivalence.

If nobody had ever annotate an NPR health article containing https://feeds.npr.org/feeds/103537970/feed.json as rel="alternate" I don't think we'd be seeing this problem, because the linking records in `document_uri` would not exist. But they do, and rewriting the database to untangle things isn't an option because we [merge](https://github.com/hypothesis/h/blob/5ab7cd380a6ccbfd08e90f438359662e58784290/h/cli/commands/normalize_uris.py#L75) equivalences.

But can we at least prevent creating new false equivalences, so that the next previously-unannotated NPR health article doesn't land in the same bucket?

One way to do that would be to stop the client from sending link elements except for `rel="canonical"`.

Currently, [here](https://github.com/hypothesis/client/blob/69c50358f82597facc4b1c4f9520b7cb8731d975/src/annotator/plugin/document.js#L231), we allow `alternate`, `canonical`, `bookmark`, and `shortlink`.

Consider two previously unannotated articles that use ``:

1 https://www.npr.org/2020/09/11/911828384/trump-says-he-downplayed-coronavirus-threat-in-u-s-to-avert-panic

2 https://www.npr.org/sections/health-shots/2020/09/11/911885577/as-covid-19-vaccine-trials-move-at-warp-speed-recruiting-black-volunteers-takes-

Annotating 1 creates document_id 22,734 and merges it with all the others.

Now I'll annotate 2 using a client that shortens the array of allowed link elements from ['alternate', 'canonical', 'bookmark', 'shortlink'] to ['canonical']. We get a fresh document_id, 1,125,647.

An alternate solution would be to keep sending these links and stop the equivalence on the back end. The client-side solution is easiest for me to demo, so that's what I'm illustrating here.