Add missing `permission_classes` / `get_permissions` to views

[khvn26]

The views listed below need to be updated with permission_classes / get_permissions.

• api/integrations/slack/views.py:SlackGetChannelsViewSet — potentially leaks Slack channels accessible to previously created integration

The following views are protected by admin login but have no permissions at class or action level:

• api/sales_dashboard/views.py:OrganisationList
• api/sales_dashboard/views.py:EmailUsage