Fix timeline markers in Firefox (regression from #13887) (#13889)

Fix timeline markers in Firefox (regression from #13887) (#13889)

Unfortunately, Firefox does not support the `keepalive` parameter
I used in the previous PR. However it supports the `navigator.sendBeacon`
API that allows that kind of things, but does not allow setting headers.

Therefore, this PR replaces it with a `sendBeacon` call that passes the
bearer token in the POST data.

Doorkeeper will then handle the auth token out of the box, as long as
it is passed as form data. Passing the query as JSON does not work.