Subfleet Hubs / Base Airports (#978)

* Update SubFleet Model

Add hub_id for storing subfleet's main base/hub airport, define a relationship with airport model to get details of the base/hub airport when needed.

* Update Admin / SubFleetController

Added the ability to read and pass hub airports to admin view for create/edit options.

* Update Admin/SubFleets.fields.blade

Added the dropdown for adding/editing main base/hub of a subfleet.

* Add migration for the hub_id column

Co-authored-by: Nabeel S <nabeelio@users.noreply.github.com>
Co-authored-by: Nabeel Shahzad <nshahzad@live.com>

app/Database/migrations/2021_01_17_044305_add_hub_to_subfleets.php

@@ -0,0 +1,20 @@
+<?php
+
+use App\Contracts\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+/**
+ * Add a hub to the subfleet is
+ */
+class AddHubToSubfleets extends Migration
+{
+    public function up()
+    {
+        Schema::table('subfleets', function (Blueprint $table) {
+            $table->string('hub_id', 4)
+                ->nullable()
+                ->after('airline_id');
+        });
+    }
+}