Make species sunburst on expert database pages clickable

[blakesweeney]

We now index the 'tax_string' of accessions. This lets us make the species sunburst clickable. So for example, when hovering over the 'Bacteria' ring in http://test.rnacentral.org/expert-database/gtrnadb I would like to be able to click and go to search like http://test.rnacentral.org/search?q=RNA%20tax_string:%22Bacteria%22%20and%20expert_db:%22GtRNAdb%22. We should check that tax_string and the sunburst are based of the same data.