fix: define podcast_id and platform_slug as foreign keys in podcasts_plaforms table

...e/Migrations/2020-06-05-200000_add_podcasts_platforms.php

app/Entities/Platform.php

app/Helpers/rss_helper.php

app/Models/PlatformModel.php

app/Resources/icons/social/discord.svg

modules/Admin/Controllers/PodcastImportController.php

modules/Admin/Controllers/PodcastPlatformController.php

themes/cp_admin/podcast/platforms.php

themes/cp_app/podcast/_partials/funding_links_modal.php