Refs #909

xgettext doesn't support them yet, which causes errors in the pot/po files.

https://savannah.gnu.org/bugs/?50920

updates #909

* Translated using Weblate (German and French)

Currently translated at 98.4% (260 of 264 strings)

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

* Translated using Weblate (French)

Currently translated at 99.6% (263 of 264 strings)

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

Specifically for IE11.

* Add a polyfill for String.prototype.includes
* Make sure all bundled code is transpiled
* Add IE specific fix in converse-otr.js

Also, declare it in the controlbox plugin, not in core, where it's not used.