[838183] jsxc: some 404s when loading resources

After logging in to jsxc, I'm seeing 404s for the following:

• jsxc.css (/plinth/static/jsxc/libjs-jsxc/jsxc.css)
• dsa-webworker.js (/plinth/static/jsxc/libjs-jsxc/lib/otr/lib/dsa-webworker.js)