Feature selector (#4074)

* WIP

* ParityBar verification

* import from index.js

* i18n expansion & tests

* Features component

* Adapt language selector to use features

* Add features to settings view

* typo

* Convert logging

* Fix earlier merge issues resulting in test failures

* Lint failure fixes (new rules)

* Fix additional listing rules

* Re-add FormattedMessage (missing after merge), fix tests

* Fix loader overrides

* grumble: split item rendering (& test)

* grumble: allow enable/disable while testing (default on)

* grumble: move LanguageSelector below Features

* grumble: don't pass visiblity prop (& update tests)

* grumble: missing observable (onClick misbehaving)

* grumble: don't reset to defaults per session

* Fix to single store instance