Make sure that anonymous XMPP logins work in Tails

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-note_5) so this is blocked by #8573.

Related issues

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

Original created by @sycamoreone on 11307 (Redmine)

Edited Nov 22, 2019 by intrigeri

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