UX: Adjust quote button position on mobile (PR #14411)

This fixes some styling and positioning issues with the quote button (and fast edit) on mobile.

I tried to use PopperJS for the position here, but that turned out to be unhelpful, because if you set the current selection’s virtual element as the target element, Popper will update its position every time there is a change in the current selection, which cause all kinds of jittery behaviour when focus changes.

The current solution, where we set the position after the initial selection (and then don’t update it) works better. With one minor change: on mobile, we center the element when the edit form is shown so that it doesn’t overflow the window width when the selection is close to the right edge of the screen.