* Refactor shouldUpdateScroll passing

So far, shouldUpdateScroll has been manually passed down from the very top of
the React component hierarchy even though it is a static function common to
all ScrollContainer instances, so replaced that with a custom class extending
ScrollContainer.

* Generalize “press back to close modal” to any modal and to public pages

* Fix boost confirmation modal closing media modal

Fix #15258

- Change overlay background to match color of viewed image
- Add interactive reply/boost/favourite buttons to footer of modal
- Change ugly "View context" link to button among the action bar

Fixes #14018

* Fix lost focus after modal closes

Regression caused by the use of the wicg-inert polyfill

* Fix regression introduced by wicg-inert

* Catch errors to please CodeClimate

* Inserting wicg-inert in project

* Import wicg-inert in moda_root component

* Update yarn.lock

* Solving code style problems

* Removing package-lock

The modal-handling code gives back focus to the element focused when the
modal opened. However, in the case of reply confirmation, it would do so
*after* the composer code itself requested focus.