Blind SSRF in /home/testslack endpoint

Medusa is an automatic video library manager for TV shows. Versions prior to 1.0.19 are vulnerable to unauthenticated blind server-side request forgery (SSRF). The testslack request handler in medusa/server/web/home/handler.py does not validate the user-controlled slack_webhook variable and passes it to the notifiers.slack_notifier.test_notify method, then _notify_slack and finally _send_slack method, which sends a POST request to the user-controlled URL on line 103 in /medusa/notifiers/slack.py, which leads to a blind server-side request forgery (SSRF). This issue allows for crafting POST requests on behalf of the Medusa server. Version 1.0.19 contains a fix for the issue.