Refactor editor placeholder translation setup
Instead of waiting for one second before updating the placeholder, only
call updatePlaceholder after the new translations have been loaded.
Also call updatePlaceholder on page initialization, making the
equivalent code in editor initialization unnecessary.
Get the DOM elements to update from the editor’s CodeMirror variable, in
case there’s more than one editor on the page.
Set the CodeMirror placeholder option in addition to directly updating
the DOM element, just in case CodeMirror reinitializes the DOM element.