[change] Make disabled organizations readonly but deletable

Is your feature request related to a problem? Please describe.

When an organization is disabled (is_active = False), writes against objects related to that organization are not consistently blocked.

Some paths already behave correctly through membership filtering, but that is not a reliable or explicit protection. A few write paths still bypass it, and disabled organizations can also appear in organization selection widgets.

Describe the solution you'd like

Make disabled organizations a first-class write guard.

Objects related to a disabled organization should:

• remain readable
• remain deletable
• be blocked from creation
• be blocked from update

In addition, organization selection fields, autocomplete widgets, and other FK pickers should exclude disabled organizations.

This behavior should be enforced consistently across:

• Django admin
• DRF views and permissions
• model validation for membership and ownership records
• queryset helpers used by organization selectors and autocomplete fields

Goals

• make the disabled-organization rule explicit instead of relying on indirect membership filtering
• provide reusable admin and DRF mixins that downstream apps can adopt
• provide a shared helper/queryset pattern for active organizations and memberships
• ensure the rule applies uniformly, including for superusers unless a view explicitly opts out
• keep read and delete operations available so disabled-organization data can still be audited and cleaned up
• keep organization lifecycle operations working, including re-enabling a disabled organization
• surface clear, translatable error messages instead of silent failures or empty querysets

Deliverables

• reusable Django admin and DRF mixins for guarding writes on disabled-organization objects
• model-level validation for membership and ownership records
• updates to organization FK/autocomplete querysets so disabled organizations are excluded from selection
• migration of existing openwisp-users call sites to the new mixins/helpers
• clear error messages when a write is blocked

Notes

• This issue is about write protection, not authentication or organization lifecycle changes.
• Existing logic that intentionally shows disabled organizations for auditing or lifecycle management should remain available where needed.
• The change should be implemented in a way that downstream OpenWISP apps can reuse.

Reference: OpenWISP Device Operation Analysis