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.