Created an admin view for Categories and Groups

[jonquet]

Issue

There is currently no graphic way to edit/create Categories and Views in OntoPortal.
It has to be done via the API calls (PATCH, PUT, DELETE) on the respective endpoints:
http://data.agroportal.lirmm.fr/categories
http://data.agroportal.lirmm.fr/groups

Proposition

Create a page in the Administration section to edit/create Categories and Views (similar to what we have for "Ontology administration" or on AgroPortal for "User Administration").

Related to this is #1 which concerns the creation of a modal to simply view the Categories and Groups.

[syphax-bouazzouni]

Done in agroportal/project-management#422, move this issue to be ready to be merged state.