allow css/js asset directory to be relocated (#13053)

This allows the directory where CSS/JS asset collections are
written to be changed, in case SERVERROOT is not writeable. Note
it does *not* allow the expected URL to be changed: whatever
directory is used, the server must be configured to serve it
at WEBROOT/assets. It may be possible to add another config
parameter to allow the admin to specify a custom asset URL,
but I thought I'd keep the first implementation simple.