Complete base flow for TOTP setup

- Includes DB storage and code validation.
- Extracted TOTP work to its own service file.
- Still needs testing to cover this side of things.

tests/Auth/MfaTotpTest.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace Tests\Auth;
+
+use Tests\TestCase;
+
+class MfaTotpTest extends TestCase
+{
+    // TODO
+}