Facebook share button links to incorrect URL

[craigstanton]

The Facebook button on the Share panel currently links to "https://niwa.github.io%2f" despite the meta tag in index.html indicating "https://niwa.github.io/tsunami-db". This current link goes to a 404 page and therefore the preview that should appear in the Facebook post is an error message.

This may be fixed when we start using a custom subdomain but it would be good if it could work when in a subwoofer such as /tsunami-db too.

[tmfrnz]

The share panel renderer currently assumes the website hosted on root
https://github.com/niwa/tsunami-db/blob/master/app/components/app/AppView.js#L712

Fix would require adding
+ window.location.pathname + '/'
here