2FA support

In a ticket for gotify server it was stated that 2FA should be added via external services: https://github.com/gotify/server/issues/461

This works well for the WebUI (I'm personally using traefik + authelia for 2FA in front of gotify).

Unfortunately this is a problem for the Android client application, which is not able to follow the redirect workflow imposed by 2FA.
This seems to be a general problem I've seen for many other selfhosted projects that provide a WebUI + client - when adding external 2FA, usually the client application breaks.

So it seems there are 3 "solutions" to this dilemma:

* Try to make the client "redirect / 2FA aware" (is this even possible in a good way? since all 2FA stuff happens on the initial http requests and therefore "outside" the API level)
* Re-think the decision not to have "native" 2FA built into the server (which I guess would be easier to follow/implement client-side)
* "Just accept" / maybe somewhere document that 2FA + Client app is not supported

Any thoughts on this / am I missing something?