Update genes route to support multiple genes

[alancleary]

The new genes route for the combined search/multi view is currently derived from the search route, meaning it only supports one gene-source pair. Update this to support arbitrarily many gene-source pairs.

[alancleary]

It's worth noting that the "arbitrarily many gene-source pairs" is derived from the deprecated multi view, but is not the same. In the multi view, the URL would contain arbitrarily many gene names and the view would resolve which source each gene belonged to and load the appropriate data. This, however, did not update the URL to specify which source each gene belonged to, meaning each time the view was loaded, the genes' sources would have to be re-resolved. This functionality will now be considered a search and should be reimplemented as part of #212. Specifically, a successful multi gene search will yield a list of genes AND corresponding sources that will be passed to the genes route, as outlined in this issue.

[alancleary]

Support for arbitrarily many gene-source pairs was added in commit bf79edb via URL matrix parameters. For example, you would load multiple genes from both the "lis" and "legfed" sources as follows: /gene;lis=gene1,gene2,gene3;legfed=geneA,geneB,geneC

Regarding my previous comment on the legacy multi URL and gene searches: these two should not be conflated. Legacy support will be added for the multi URL (see #218) by redirecting to the new matrix parameter URL and passing every gene to every source. If two or more sources contain one of the gene names, a track will be loaded from each such source, as was previously done. The act of actually determining what source(s) a gene name belongs to is a search and will be implemented separately (see #212).