Fix incoherent tag handling

MR description

This MR adresses #471 (closed): change behaviour of dataset tags when adding or modifying a dataset, to make this behaviour coherent:

• modify DatasetManager functions
  • for adding a dataset (add_database) to properly check tags using a new dedicated function (search_conflicting_tags). This fixes the CLI
  • for updating a dataset (modify_database_info). This function was unused, so we changed the prototype to make it match the needs of the GUI. Also added proper checks with search_conflicting_tags.
• fix GUI to do the checks. As GUI currently does not properly use DatasetManager (see #338) this requires some distinct fixes. We changed the Flask server routes for adding/updating datasets to use the DatasetManager (minimal refactoring needed to fix the issue)
• adapt and add unit tests

What can be tested, under both CLI and GUI:

• add a dataset with same tags as an existing dataset (conflict)
• add a dataset with subset of tags from an existing dataset (conflict)
• add a dataset with superset of tags from an existing dataset (conflict)
• add twice the default dataset with same tags (conflict)
• update tags from a dataset to be same tags as an existing dataset (conflict)
• update tags from a dataset to be subset of tags from an existing dataset (conflict)
• update tags from a dataset to be superset of tags from an existing dataset (conflict)
• several scenarios of adding dataset with some tags common with existing datasets, but not all (ok)
• ...

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