Assistant settings reorganization

[julien-nc]

Note

This is a draft, here for reference and so that we can discuss.
Not to be implemented yet, not all library components required for the changes are available yet.

Reorganized settings using the new form components. See nextcloud/server#55667

[julien-nc]

@kra-mo Would you consider that the Assistant's settings adjustments are done with #393 ?

[kra-mo]

Things can always be improved but as far as adopting the new components is concerned, I think so, yes.