Full coverage for JS/Ajax functionality

This module relies heavily on Javascript and Ajax but until recently it had no Selenium test coverage. In 🐛 Add some FunctionalJavascript tests + fix JS error on inbox init Active we've introduced few JS tests. But the JS functionality continue to be quite unstable and prone to errors. We don't have 100% JS test coverage. There are also some tasks planned, such as as 📌 Drop jQuery dependency. Convert to vanilla JS Active that are quite risky and they might break things without proper test coverage.

Ensure full Selenium test coverage for all functionalities that are JS/Ajax based.