BUG: Fix add new button not passing the correct parent ID.

The `%s` parent ID placeholder was being URL encoded, so it was not being
replaced properly when adding a new page. This fix separately adds
placeholder parameters to the URL to avoid this issue.

The fix is not ideal, but there is no easy way to indicate that some
URL parameters should be URL encoded and others should not while still
correctly constructing the URL.
Merge pull request #870 from ajshort/pull-1

BUG: Fix add new button not passing the correct parent ID.