While adding support for MUCs, I refactored converse-omemo somewhat to move functions
out of `overrides` and to use async/await

Updates #1180

updates #1180

Currently translated at 99.5% (367 of 369 strings)

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

Currently translated at 100.0% (369 of 369 strings)

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

We don't need an event handler that sets the "opposite" features to
false if we set all features properly in getFeatures

Stop listening on the _converse obj sooner

Currently translated at 55.4% (204 of 368 strings)

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

Currently translated at 100.0% (368 of 368 strings)

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

Otherwise trimChats called in `fullscreen` tests (due to being
registered in a previous test)

And call it when `converse.initialize` gets called again

So simplify the code that saves the window state

When OMEMO isn't possible, render it as grey.

This change makes OMEMO for MUCs easier, since there I anticipate that
OMEMO support might change dynamically based on who enters/exits the
room.

updates #1180