feat: Organisations admin screen with the avatar and the members manager

- controller golem15.user.organisations behind golem15.users.access_users
- list and form (name, slug preset from the name, description, avatar)
- members relation manager: link sets and unlink clears users.organisation_id
- deleting an organisation releases its members first
- Organisation gains Fillable, Rules, MorphName, AttachRelations, BeforeValidate
  and the Members relation; models.Slugify
- navigation side item organisations
- TestAdminOrganisations, TestAdminOrganisationMembers, TestSlugify

controllers/admin_registry.go

@@ -13,6 +13,7 @@ func AdminControllers(app func() *backpack.App) []pact.AdminController {
 	return []pact.AdminController{
 		usersAdminController{app: app},
 		usergroupsAdminController{app: appAccess(app)},
+		organisationsAdminController{app: appAccess(app)},
 	}
 }
 
@@ -47,4 +48,13 @@ func init() {
 	var _ pact.FormBeforeDelete = usergroupsAdminController{}
 	var _ pact.FormAfterDelete = usergroupsAdminController{}
 	var _ cabana.PermissionEditorProvider = usergroupsAdminController{}
+
+	var _ pact.AdminController = organisationsAdminController{}
+	var _ pact.AdminPermissioned = organisationsAdminController{}
+	var _ pact.AdminRecordSource = organisationsAdminController{}
+	var _ pact.FormBeforeCreate = organisationsAdminController{}
+	var _ pact.FormBeforeUpdate = organisationsAdminController{}
+	var _ pact.FormBeforeDelete = organisationsAdminController{}
+	var _ pact.RelationExtendManageQuery = organisationsAdminController{}
+	var _ cabana.AdminRelationContractProvider = organisationsAdminController{}
 }