fixes url tokenization with respect to wwwroot when elgg is installed in...

... a subdirectory to prevent path duplication

Without this change, if elgg is installed in a directory eg. [domain]/elgg
Adding a new menu item would tokenize the url as [wwwroot]elgg/path/to/page
And the resulting menu item would point to [domain]/elgg/elgg/path/to/page
Note the duplication of /elgg