Add categories to checks

[corentinbettiol]

Currently, all checks are processed in the same way and are displayed in a large list.
A cool thing to add could be a category tag for checks, which could add readability to lists.

The category will be accessible via the Site instance in each check, and it will be of the form "retrieve the list according to its name, or create a new list with the new name".

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Old display:

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New display: