Revert "fix the urljoins -- was crashing due to an Undefined object"

This reverts commit 0e091105d17e1d26d064947a1b09a2a13c732748.
Revert "fix #986 — strip leading slashes from joined URLs to make nikola-in-a-subdirectory work"

This reverts commit 87bb8b722cad3f805ebfcd3ee0c130b4522297bc.