feat(rss): add podcast-namespace tags for platforms + previousUrl tag

- add podcast:id tag
- add podcast:funding tag
- add podcast:social tag
- add podcast:previousUrl tag
- add missing platforms with icons
- update platforms table to include social and funding platforms
- rename platform_links table to podcasts_platforms
- move podcast import methods from podcast controller
- update import functionality to insert platforms from rss

closes #73, #75, #76, #80

app/Controllers/Platform.php

@@ -0,0 +1,24 @@
+<?php
+
+/**
+ * @copyright  2020 Podlibre
+ * @license    https://www.gnu.org/licenses/agpl-3.0.en.html AGPL3
+ * @link       https://castopod.org/
+ */
+
+namespace App\Controllers;
+
+use CodeIgniter\Controller;
+
+/*
+ * Provide public access to all platforms so that they can be exported
+ */
+class Platform extends Controller
+{
+    public function index()
+    {
+        $model = new \App\Models\PlatformModel();
+
+        return $this->response->setJSON($model->getPlatforms());
+    }
+}