Europe PMC document equivalence issue

## Bug report form
### Steps to reproduce
1. Go to https://europepmc.org/article/MED/24806729
2. Click on 'PubMed Commons archived comments (3)' under the article main title

### Expected behaviour
It should open the hypothesis sidebar and show 3 notes for the group 'Europe PMC'.

These are the three annotations that should be displayed: https://hypothes.is/groups/NMb8iAjd/europe-pmc?q=PMID%3A24806729

### Actual behaviour
No annotations are shown for the group 'Europe PMC'.

### Additional details

In 2019, EuropePMC changed the URLs pattern.

They have a redirection from `[...]/abstract/MED/[...]` to `[...]/article/MED/[...]`.

This is a document equivalence issue.

The search API should return 3 annotations for these two queries:
* https://hypothes.is/api/search?uri=https://europepmc.org/article/MED/24806729 (returns 0)
* https://hypothes.is/api/search?uri=https://europepmc.org/abstract/MED/24806729 (returns 3)

Something that could obscure this issue, but it is not directly related, is that:
* Europe PMC modified the Hypothesis client to disable login
* There are duplicates annotations in the 'Public' Group.

https://hypothes-is.slack.com/archives/C2BLQDKHA/p1602161751190300