landing page organism viz (#289)

[d-callan]

working our way towards a viz for the landing page. my understanding is that this pr should do the following:

• provide a summary of what organisms/ assemblies we currently support as a sunburst
• for segments of the sunburst w ncbi tax ids that match our 'taxonomic group' filter, allow a user to select that segment and go to a filtered organism/ assembly page
• be pretty lol
• automagically produce the required data via ncbi api calls

resolves #289

[dannon]

Here's the current view of it as demo'd in the call today;

Right panel is reactive and shows information about the currently selected node. Quick links (organism page, etc.) and other stuff forthcoming, suggestions welcome.

In the screenshot the currently selected node is the root, which won't have relevant info to show. We should instead show some intro text like "Click an area of interest in the visualization to..." etc.

[dannon]

Note to myself that the 'happy accident' of parent nodes including themselves in the count, causing the organic-looking decreasing arc size is visually interesting but I should fix the count.