This function should take a scope or a set of ids. The way it is now it will apply to *all* textarea fields, including those that already are expandable. I really don't like the implementation of `expandable` which creates one very ugly invisible DOM element for *every single* textarea field that it applies to. Now, whenever `Libertree.UI.makeTextAreasExpandable` is called, the number of *additional* DOM elements is equal to the number of loaded posts. We should exclude posts that already are expandable.