Determines whether we show the icon for opening the client's "About" modal.

updates #1373

* CSN tests don't belong under "Groupchats" section tests.
* Move modal tests to their own grouping

Also include the unminified versions for people who wants to modify
them.

* `connectionInitialized` and `converse-loaded` events
* allow_login` setting

* Add new test helper to create contacts via IQ exchange
* Use roster nickname to set message fullname

When a new contact is added, check if we already have a chatbox open for it,
and if so attach it to the chatbox.

Currently translated at 100.0% (372 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/es/

Currently translated at 60.5% (225 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/pt_BR/

Currently translated at 82.3% (306 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/pl/

Currently translated at 55.6% (207 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/it/

Currently translated at 76.3% (284 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/he/

Currently translated at 100.0% (372 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/de/

Currently translated at 100.0% (372 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/fr/

Currently translated at 100.0% (372 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/nl_BE/

Currently translated at 100.0% (372 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/nl/

Currently translated at 100.0% (372 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/cs/

Currently translated at 28.5% (106 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/zh_Hans/

Currently translated at 100.0% (372 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/af/

Currently translated at 82.3% (306 of 372 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/pl/

Fixes scrollbar appearing next to spinner when logging in.

* Replace `tearDown` overrides with event handlers.
* Move presence listeners to `connected` event handler, otherwise they
  don't get called when logging in again after disconnection.
* Add `clearSession` listeners to clean up upon disconnection.
* Use `Promise.all` instead of listening for multiple events.