Simplified guard names and rolled out guard route checks

- Included tests to cover for LDAP and SAML
- Updated wording for external auth id option.
- Updated 'assertPermissionError' test case to be usable in BrowserKitTests

app/Http/Middleware/CheckGuard.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace BookStack\Http\Middleware;
+
+use Closure;
+
+class CheckGuard
+{
+    /**
+     * Handle an incoming request.
+     *
+     * @param  \Illuminate\Http\Request  $request
+     * @param  \Closure  $next
+     * @param string $allowedGuards
+     * @return mixed
+     */
+    public function handle($request, Closure $next, ...$allowedGuards)
+    {
+        $activeGuard = config('auth.method');
+        if (!in_array($activeGuard, $allowedGuards)) {
+            session()->flash('error', trans('errors.permission'));
+            return redirect('/');
+        }
+
+        return $next($request);
+    }
+}