Modalities to folders mapping through GUI for MedicalFolderDataset

Closes #320 (closed). Should not be merged before !132 (merged)

This MR introduces the possibility to add a mapping of modality names to local folder names when adding a MedicalFolderDataset. It also shows a general pattern that could be copied to introduce other DataLoadingPlans/DataLoadingBlocks in the future.

Includes:

• editing modalities to folder mapping in the GUI
• saving modalities to folder mapping in a DLP, when adding the dataset
• loading and reusing DLP defined for a previous dataset
• avoid duplicate DLP: don't save DLP if reusing an existing DLP without modifying it; remove DLP if dataset add fails
• enhanced coherency checks before adding dataset

Not included:

• support for DLP via CLI (#340)
  • nota: commented out current CLI code for DLP in node/cli_utils/_medical_folder_dataset.py function def add_medical_folder_dataset_from_cli() that does not work with final version of DLP
  • nota: commented out some tests in test_cli.py and test_dataset_manager.py that need DLP support in CLI
• page to view and delete DLP (#341 (closed))
• DLP with a data_type to authorize only loading of DLP done with the same type of dataset

Guidelines for the review:

• @scansiz: as discussed, some styling needs to be modified (component aspects, layering of text)
• @scansiz: made intensive use of getState(), maybe needs better solution ?
• javascript coding style may need not be good
• plus classical review (bugs ...)