set backend and version env

src/Common.js

@@ -15,7 +15,7 @@ import renderMd from './Markdown';
 
 export { format_time, Time, TitleLine };
 
-export const API_BASE = '/_api/v1';
+export const API_BASE = `${process.env.REACT_APP_BACKEND || '/'}_api/v1`;
 
 // https://stackoverflow.com/questions/3446170/escape-string-for-use-in-javascript-regex
 function escape_regex(string) {