Added migration 0.8 for Neo4j database (see commit's body)

Migration 0.8 update every node having an 'url' property and
remove this property to create a new 'reference' property
with data consistency.
Added node type checking in migration-0.8
Updated contribute template to list every field sources
Started ContributeCtrl handling of multiple sources
Added separted path_sources method for IndividualRessource
Replaced url property by reference property on FieldSource model
Merge branch 'develop' into feature/MultipleSources

Conflicts:
	app/detective/individual.py
Simplified patch_sources method of individual ressource
Added special styles for delete-source buttons
Implemented multiple sources on contribute interface
Simplified contribute styles
Implemented multiple sources edition/creation
Improved (a lot) multiple source edition style
Improved UX of source list: we now dont lose field focus on source delete
Removed console.log from focusOn directive
Created & implemented new popover source list
Included new source popover files in home.dj.html & base.less
Refactored ContributeCtrl -> externalized isValidURL method to UtilsFactory
Added usage of topic.single.sources directive into topic.single template
Implemented multiple sources on IndividualSingleCtrl