javascript: autosave with explicit cookies settings

Older browsers implemented a different default value for `window.fetch`
credentials: it was omitted unless explicitely set.

So we force the value for these older browsers:

- Firefox 39-60
- Chrome 42-67
- Safari 10.1-11.1.2

See https://github.com/github/fetch#sending-cookies

app/javascript/new_design/autosave-controller.js

@@ -37,6 +37,7 @@ export default class AutosaveController {
       const fetchOptions = {
         method: form.method,
         body: formData,
+        credentials: 'same-origin',
         headers: { Accept: 'application/json' }
       };