Toggle task types in admin settings

[julien-nc]

It would be nice to let the admins toggle each task type in the admin settings.
Since the task type list is dynamic, task types we don't know would be enabled by default.
The setting value could be stored as a JSON object string.
A disabled task type should not be displayed in the assistant modal.

Let's do that in the server's AI admin settings and not in the assistant admin settings.