Fix leading slash in path which creates a problem for S3 when URL path has double slashes

[mbocevski]

S3 throws an access denied when URL has doubleslashes in it, and having a leading slash in templates causes an access denied error. For example: https://mydjangosite.com//jquery/ajax.js would not work.

This fix removes a leading slash from the path and can be expanded to remove all double-slashes, however I feel that removing the leading slash is enough for most common scenarios.