Use relative URLs throughout the UI

To allow serving from a subdirectory using a reverse proxy.

ui/provisions-page.ts

@@ -114,7 +114,7 @@ const formData = {
 const getDownloadUrl = memoize(filter => {
   const cols = {};
   for (const attr of attributes) cols[attr.label] = attr.id;
-  return `/api/provisions.csv?${m.buildQueryString({
+  return `api/provisions.csv?${m.buildQueryString({
     filter: stringify(filter),
     columns: JSON.stringify(cols)
   })}`;