Added sub-bookings model and migration.

app/SubBooking.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace App;
+
+use Illuminate\Database\Eloquent\Model;
+
+class SubBooking extends Model
+{
+    //
+}

database/migrations/2018_09_07_102728_create_sub_bookings_table.php

@@ -0,0 +1,61 @@
+<?php
+
+use Illuminate\Support\Facades\Schema;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Database\Migrations\Migration;
+
+class CreateSubBookingsTable extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @return void
+     */
+    public function up()
+    {
+        Schema::create('sub_bookings', function (Blueprint $table) {
+            $table->increments('id');
+            
+            // The reference which describes the sub-booking
+            $table->string('reference');
+
+            // The amount of the subbooking
+            $table->float('amount', 8, 2);
+
+            // If the sub-booking belongs to the target of a person, this target
+            // is referenced here
+            $table->integer('person_target_id')->unsigned()->nullable();
+            $table->foreign('person_target_id')->references('id')->on('person_targets')
+                  ->onDelete('set null');
+
+            // Each sub-booking belongs to one (super-)booking. In the most
+            // simple case, a booking consists of exactly one sub-booking.
+            $table->integer('booking_id')->unsigned();
+            $table->foreign('booking_id')->references('id')->on('bookings')
+                  ->onDelete('cascade');
+
+            // The account type to which the sub-booking belongs
+            $table->integer('account_type_id')->unsigned();
+            $table->foreign('account_type_id')->references('id')->on('account_types')
+                  ->onDelete('restrict');
+
+            // If the sub-booking belongs to a project, this project
+            // isreferenced here.
+            $table->integer('project_id')->unsigned()->nullable();
+            $table->foreign('project_id')->references('id')->on('projects')
+                  ->onDelete('set null');
+
+            $table->timestamps();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     *
+     * @return void
+     */
+    public function down()
+    {
+        Schema::dropIfExists('sub_bookings');
+    }
+}