Support multiple publications associated with an experiment

### Context

During the LowFi Search Page design review (documented [here](https://github.com/AlexsLemonade/refinebio/issues/216#issuecomment-385702057)), we discussed a case where multiple publications cite an experiment/accession. For example, the GEO accession page for [GSE49711](https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE49711) displays three publications.

### Problem or idea

Currently, we're only getting the _first_ publication associated with an experiment. We'd want to capture the relationship between the experiment and _all_ publications.

### Solution or next step

This ticket will need to be fleshed out with implementation details at a later date (post Keytar Kurt).

### Related issues

It probably makes sense to tackle this around the same time as #214.

### New Issue Checklist

- [x] The title is short and descriptive.
- [x] You have explained the context that led you to write this issue.
- [x] You have reported a problem or idea.
- [x] You have proposed a solution or next step.

Review the context in issue #216 and the related issue #214, using GEO accession GSE49711 as the example case. The implementation details are not yet defined; done means capturing the relationship between an experiment and all associated publications rather than only the first.

Written by the indexing model from the issue text.