fix #1560 Load tutorial from json in tutorial model

Previously, offline tutorial load tutorial in views.py
but don't get licence with licence code in manifest
of the tutorial concerned. This commit use load_json
method in tutorial class which load entierly the
tutorial.
docs #1560 Improve tests of tutorial module

Improve tests of tutorial module to compare
code licence and not title licence.
Merge pull request #2 from GerardPaligot/fix_1560_QA

QA for your fix of issue #1560 in zds-site project