Include attribution link with gisaid_epi_isl

[trvrb]

Description of proposed changes

This updates behavior to match what we were doing for genbank_accession. Here, we special case both genbank_accession and gisaid_epi_isl and construct direct DB links when these are present rather than using the more generic accession / url metadata pair.

This update allows users to lookup attribution info for individual samples even if workflow metadata did not include this attribution data.

Testing

Tested locally.

[jameshadfield]

Tested using a variety of (core) datasets to check the different formats data may appear in (partly to remind myself!)

Fields	example	behavior
(none)	H7N9	Nothing displayed
url	locally tested	"Strain URL" displayed with link to supplied url
accession	H3N2	Displayed without any links
accession, url	Historical nCoV dataset	display the accession which is a link to the specified URL
genbank_accession, url	ncov/open	"Genbank Accession" displayed, with link constructed from the accession number. The provided url information is ignored.
gisaid_epi_isl, url	ncov/gisaid	"GISAID EPI ISL" displayed, with a link to the epicov JSON. The provided url information is ignored.
genbank_accession, gisaid_epi_isl, url	ncov/gisaid	"Genbank accession" and "GISAID EPI ISL" displayed, each with a link as above. The provided url information is ignored.

@trvrb if this is as you expect it to be, please merge & I can release in this week's auspice release.

[trvrb]

Thanks for outlining James. We may want to rethink how we do accessions at some point to make it easier to have multiple accessions with matched URLs for a sample, but I think this current priority system is good for the moment. genbank_accession and gisaid_epi_isl take precedence over more generic accession / url inclusion.