Add "sources" to gene summaries TSV

[gaberudy]

Hi there, I'd do a pull request if I was a bit more familiar with the backend data model, but I was wondering if we could get the list of sources sited in the CiVIC front-end for each gene in the gene summaries tsv.

It looks like modifying:

https://github.com/griffithlab/civic-server/blob/988493589f214d1a588d28d1f850f3a2766df250/app/presenters/gene_tsv_presenter.rb

to include the "sources" field in the gene model.

Really the PMID for each source is what I'm going for, but I can see that in the web UI:

https://civicdb.org/events/genes/5/summary/variants/579/summary

there are two references objects, each being a publication which has this type of data, but if that could be mapped back to a list of PMIDs, that would be fantastic!