Add the CSP headers

Squashed commit of the following:

commit 468945826621d2e586f7e5fa773623c4accc316a
Author: Jonny Barnes <jonny@jonnybarnes.uk>
Date:   Fri Mar 2 16:42:30 2018 +0000

    Update changelog

commit 36c6edce091c41861879a982e6ad250b395abbcf
Author: Jonny Barnes <jonny@jonnybarnes.uk>
Date:   Fri Mar 2 16:42:23 2018 +0000

    Add a test

commit ef9d7b564f8ea4f4528c42f411c14ddfaa132082
Author: Jonny Barnes <jonny@jonnybarnes.uk>
Date:   Fri Mar 2 16:42:13 2018 +0000

    Apply the CSPHeader middleware to all `web` requests

commit 737bfca3a6b446d52c0d0a8cc1b7b1c422876c0b
Author: Jonny Barnes <jonny@jonnybarnes.uk>
Date:   Fri Mar 2 16:41:45 2018 +0000

    Add a CSP header to a response, as well as the Report-To header

tests/Feature/CSPHeadersTest.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Tests\Feature;
+
+use Tests\TestCase;
+
+class CSPHeadersTest extends TestCase
+{
+    /** @test */
+    public function check_csp_headers_test()
+    {
+        $response = $this->get('/');
+        $response->assertHeader('Content-Security-Policy');
+        $response->assertHeader('Report-To');
+    }
+}