♻️ refacto needed on all converse plugins

[roro-lv]

The code in converse plugins (utils/conversejs/converse-plugins/...) lacks of clarity and is a bit confused. It need to be refactored. Here are some suggestion to start :

• stanzas are typed with HTLMElement, which works, but which isn't reflecting the reallity of it. Indeed, stanza are xml-written and have nothing to do with HTML. It should be changed to use correct typing.
• stanzas are processed with functions as getElementsByTagName or getAttribute. A proper xml lib should be installed to easily parse stanzas.
• Some comments could be adeed, at least to explain what stanzas are supposed to contain