Ensure DeleteUser is not allowed to Delete Orgs and visa versa (#10134)

* add check to DeleteUser

* add check to DeleteOrganization

* add Test

* remove redundancy (deleteOrg is only used in DeleteOrganization)

* Update models/org.go

Co-authored-by: zeripath <art27@cantab.net>

models/org_test.go

@@ -272,8 +272,8 @@ func TestDeleteOrganization(t *testing.T) {
 	assert.Error(t, err)
 	assert.True(t, IsErrUserOwnRepos(err))
 
-	nonOrg := AssertExistsAndLoadBean(t, &User{ID: 5}).(*User)
-	assert.Error(t, DeleteOrganization(nonOrg))
+	user := AssertExistsAndLoadBean(t, &User{ID: 5}).(*User)
+	assert.Error(t, DeleteOrganization(user))
 	CheckConsistencyFor(t, &User{}, &Team{})
 }