Make sure that anonymous XMPP logins work in Tails

Originally created by @sycamoreone on #11307 (Redmine)

Before trying to find/organize a XMPP server that supports anonymous logins, we should make sure that these actually work in the XMPP client shipped in Tails.

It does not work in Pidgin (#11307 (comment 60911)) so this is blocked by #8573.

Related issues

Related to #8573
Has duplicate #11701 (closed)
Blocks #11317 (closed)

Edited May 15, 2020 by sycamoreone

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