Remove notebook canonical path constraint
Remove notebook canonical path constraint

Per discussion on Gitter.im, the requirement that notebook files contain
the canonical path to the web root as a prefix prevents symbolic linking
of notebook files or directories into an installation:
  https://gitter.im/andypetrella/spark-notebook?at=56a9b9888fbaf4220af9c8e4

This PR removes that check to allow symlinks from other parts of a
filesystem.

spark-notebook/spark-notebook,minyk/spark-notebook,maasg/spark-notebook,0asa/spark-notebook,0asa/spark-notebook,antonkulaga/spark-notebook,andypetrella/spark-notebook,radek1st/spark-notebook,maasg/spark-notebook,maasg/spark-notebook,antonkulaga/spark-notebook,antonkulaga/spark-notebook,minyk/spark-notebook,0asa/spark-notebook,radek1st/spark-notebook,deanwampler/spark-notebook,deanwampler/spark-notebook,radek1st/spark-notebook,andypetrella/spark-notebook,antonkulaga/spark-notebook,andypetrella/spark-notebook,andypetrella/spark-notebook,deanwampler/spark-notebook,minyk/spark-notebook,spark-notebook/spark-notebook,deanwampler/spark-notebook,maasg/spark-notebook,minyk/spark-notebook,spark-notebook/spark-notebook,spark-notebook/spark-notebook,radek1st/spark-notebook,0asa/spark-notebook