Finished off script CSP rules

- Added caching for custom html head parsing to add nonce.
- Also moved api docs page into web routes to prevent issues.

app/Util/CspService.php

@@ -0,0 +1,72 @@
+<?php
+
+namespace BookStack\Util;
+
+use Illuminate\Support\Str;
+use Symfony\Component\HttpFoundation\Response;
+
+class CspService
+{
+    /** @var string */
+    protected $nonce;
+
+    public function __construct(string $nonce = '')
+    {
+        $this->nonce = $nonce ?: Str::random(16);
+    }
+
+    /**
+     * Get the nonce value for CSP.
+     */
+    public function getNonce(): string
+    {
+        return $this->nonce;
+    }
+
+    /**
+     * Sets CSP 'script-src' headers to restrict the forms of script that can
+     * run on the page.
+     */
+    public function setScriptSrc(Response $response)
+    {
+        if (config('app.allow_content_scripts')) {
+            return;
+        }
+
+        $parts = [
+            '\'nonce-' . $this->nonce . '\'',
+            '\'strict-dynamic\'',
+        ];
+        $value = 'script-src ' . implode(' ', $parts);
+        $response->headers->set('Content-Security-Policy', $value, false);
+    }
+
+    /**
+     * Sets CSP "frame-ancestors" headers to restrict the hosts that BookStack can be
+     * iframed within. Also adjusts the cookie samesite options so that cookies will
+     * operate in the third-party context.
+     */
+    public function setFrameAncestors(Response $response)
+    {
+        $iframeHosts = $this->getAllowedIframeHosts();
+        array_unshift($iframeHosts, "'self'");
+        $cspValue = 'frame-ancestors ' . implode(' ', $iframeHosts);
+        $response->headers->set('Content-Security-Policy', $cspValue, false);
+    }
+
+    /**
+     * Check if the user has configured some allowed iframe hosts.
+     */
+    public function allowedIFrameHostsConfigured(): bool
+    {
+        return count($this->getAllowedIframeHosts()) > 0;
+    }
+
+
+    protected function getAllowedIframeHosts(): array
+    {
+        $hosts = config('app.iframe_hosts', '');
+        return array_filter(explode(' ', $hosts));
+    }
+
+}