feat: add invite notification emails, functionality to admin dashboard and sign up page

Signed-off-by: Miguel Nogueira <me@nogueira.codes>

app/Http/Requests/InvitationRequest.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace App\Http\Requests;
+
+use App\Facades\Options;
+use Illuminate\Auth\Access\AuthorizationException;
+use Illuminate\Foundation\Http\FormRequest;
+
+class InvitationRequest extends FormRequest
+{
+    public function rules(): array
+    {
+        return [
+            'email' => ['required', 'email', 'max:254'],
+        ];
+    }
+
+    public function authorize(): bool
+    {
+        if (Options::getOption('enable_registrations')) {
+            return false;
+        }
+
+        return true;
+    }
+
+    protected function failedAuthorization()
+    {
+        throw new AuthorizationException(__('You cannot request a new invite for this user/e-mail address right now. Keep in mind that users can only be invited once.'));
+    }
+}