http/2 support for the web UI

Graylog should natively support http/2 for the web UI.

Graylog has the tendency to make large amounts of small requests to the server. These would be slightly faster from user perspective if http/2 was in use, even without any other changes.

I think the service is run by Jersey which supports http/2. However http, and http/2 capable services seem to be different classes. I have no idea whether the http/2 capable endpoint also can speak http/1. Perhaps adding an experimental configuration option for changing the protocol implementation could be a good approach here.

I investigated serving http/2 to clients but downgrading the protocol at reverse proxy. It's a very bad idea because of the security implications related to downgrades. The whole chain handling requests should natively speak h2/h2c for security reasons.