Set the foreign key person_targets->person_id from 'on delete restrict' to 'on

delete cascade', since it should be possible to delete a person which is even
and has paid all debts.

Before showing the delete confirmation dialog, it is checked, if there is any
open target.

database/migrations/2018_09_07_090338_create_person_targets_table.php

@@ -20,7 +20,7 @@ class CreatePersonTargetsTable extends Migration
             // target
             $table->integer('person_id')->unsigned();
             $table->foreign('person_id')->references('id')->on('people')
-                  ->onDelete('restrict');
+                  ->onDelete('cascade');
 
             // The date on which the debt arises
             $table->date('date');