Add a page/input in the UI to put an ontology as flat for big resources

[syphax-bouazzouni]

See agroportal/project-management#360 for details, in summary when an ontology has a lot of concepts the classes tab or the tree view load indefinitely. A solution is to set that ontology as flat, so that we don't try to display all the concepts but tell the users to search for the concept that he want to see the details. See https://agroportal.lirmm.fr/ontologies/FLOPO?p=classes for example.

The problem right now is that we don't have a way in the UI to set that attribute flat to true. We can do it only by the API.

This issue propose to add somewhere a UI to edit that attribute.