Help Panel: Fix textarea focus on mobile when VE is activated
- detach then append the editor overlay on mobile, to avoid phantom text area
input when our process dialog is open
- remove the scroll event handler from VE target window on mobile when the CTA
is clicked, then rebind it when the help panel is closed. This is necessary
because iOS Safari erroneously triggers a scroll event on the
MobileFrontendArticleTarget surface
Bug: T212967
Change-Id: I104a5e139d58ee9995d10b67ff52bb2f52399ae8