Expose new UserGroup attribute in admin-area POST/PATCH/GET endpoints

[tcompa]

Requires:

• Store vizarr-related folder information in UserGroup attributes fractal-server#1809

The fractal-server changes are

• The UserGroup model and schemas will have a new viewer_paths string-array non-nullable attribute.
• This new attribute can be optionally included in POST /auth/groups/ request body
• This new attribute can be optionally included in PATCH /auth/groups/{group_id}/ request body
• This new attribute will always be part of response bodies for GET /auth/groups/, GET /auth/groups/{group_id}/, PATCH /auth/group/{user_id}/

The necessary fractal-web changes reflect the ones above:

• Display the number of viewer_paths for each group in the all-groups page (.../v2/admin/groups)
• Display this new attribute in the single-group page (.../v2/admin/groups/{group_id})
• Include this new attribute in the edit-page (.../v2/admin/groups/{group_id}/edit)

[tcompa]

Available as of fractal-server 2.6.3.

We also included an endpoint for a standard users to learn about their viewer_paths (GET /auth/current-user/viewer-paths/, returning a list of paths), and this information could be perhaps be displayed somewhere in the current-user's pages

I'd start simple, by just listing the relevant user pages:

1. profile (always)
2. settings (if FRACTAL_RUNNER_BACKEND is slurm or slurm_ssh)
3. viewer paths (if PUBLIC_FRACTAL_VIZARR_VIEWER_URL is set)

[zonia3000]