Change action such it is displayed for folders with default views

The Sort folder action doesn't display on folders that have a default view configured.  This pull request resolves this issue by checking either the current context if the sort view is available, and if not, then checking if the context is a default view.  If this latter condition is met, then it checks the ``folder`` object.

The URL expression is changed to always point to the current folder URL, which is the same behaviour as above -- either the current context (if a folder itself) or the parent folder, if a default view.