Fix Create New Page and Rename Page dialogs redirect behaviour

The Create New Page and Rename Page dialogs exhibit problematic redirect behaviour in some cases when the entered page name is not an absolute path.

Examples are:

* Attempting to create page B from http://localhost:4567/A redirects to B/
* Attempting to rename page to A/B from http://localhost:4567/A redirects to http://localhost:4567/rename/A 

This pull request modifies the `abspath` JavaScript function added in #598 to handle the case when the `path` argument is empty.