Autosize textboxes w/out highlighting placeholders

Scrolling within textareas on the page is a bit grim. Which is why we
don’t do it for the textboxes that people use to edit templates.

This commit will allow us to extend the auto-resizing of `<textarea>`s
to those which don’t need the highlighting of placeholders.

The code is still quite coupled to the placeholder highlighting code,
because both work by copying the content of the `<textarea>` into a
`<div>` that underlaps the textbox. This `<div>` is used for both
rendering the placeholder highlights, and calculating the natural height
of the content. So it would be hard/confusing to split the two bits of
code into separate modules.

app/templates/components/textbox.html

@@ -4,6 +4,7 @@
   hint=False,
   highlight_tags=False,
   autofocus=False,
+  autosize=False,
   colour_preview=False,
   help_link=None,
   help_link_text=None,
@@ -47,7 +48,8 @@
     %}
     {{ field(
       class=field_class,
-      data_module='highlight-tags' if highlight_tags else '',
+      data_module='highlight-tags' if highlight_tags or autosize else '',
+      data_highlight_placeholders='true' if highlight_tags else 'false',
       rows=rows|string,
       **kwargs
     ) }}