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?

Beginne damit, das verlinkte gotify/server-Issue 461 zu lesen und nachzuverfolgen, wie der Android-Client die anfängliche HTTP-Anfrage und Weiterleitungen verarbeitet. Vergleiche die drei beschriebenen Optionen: Client-Unterstützung für Weiterleitungen, native serverseitige 2FA oder Dokumentation der Einschränkung. Als erledigt gilt die Aufgabe, wenn eine Richtung festgelegt und eine klar definierte Änderung am Client oder an der Dokumentation vorhanden ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.