fix(platform): update submit url

Update submit URLs for podcasting platforms.

Update instructions

UPDATE cp_platforms
SET `submit_url`='https://podcastindex.org/add'
WHERE `submit_url`='https://api.podcastindex.org/signup';

UPDATE cp_platforms
SET `home_url`='https://music.amazon.com/', `submit_url`='https://podcasters.amazon.com/'
WHERE `slug`='amazon';