organize the code enabling functions according to the type of Node in the Tree

• Nodes have common functions and some are different
• Indeed: the function are not the same according to the type
  • For instance create a new graph for corpus but not for Node user (issue to be open soon)
  • Add new documents to an existing corpus (issue to be open soon)

Implementation expected for this issue

• Node User: create corpus button
• icon to use
• add drag-drop functionality for corpus files. Append to files_id list and set corpus_id using this backend: https://dev.gargantext.org/swagger-ui/#/Gargantext/post_api_v1_0_new