Déplacer les actions et mutations simples vers elementsv2

• Mutation set
• Mutation setElementNeighbors
• Mutation createMetadata (removed)
• Mutation updateMetadata (removed)
• Mutation deleteMetadata (removed)
• Mutation addClassification
• Mutation removeClassification
• Action get
• Action patch
• Action delete
• Action listElementNeighbors
• Action createMetadata (moved to js/helpers/metadata.js)
• Action patchMetadata
• Action deleteMetadata