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.
* Periodically save timeline markers
This saves timeline markers immediately upon message arrival, but not more
than once every 5 minutes.
This does not change how the markers are saved on closing the window,
except that it avoids submitting them if there is no need for it.
* Use the Fetch API when possible instead of XHR on window unload
