GHSA-hvw3-p9px-gpc9: Gophish before 0.12.0 vulnerable to Open Redirect

This affects the package github.com/gophish/gophish before 0.12.0. The Open Redirect vulnerability exists in the next query parameter. The application uses url.Parse(r.FormValue("next")) to extract path and eventually redirect user to a relative URL, but if next parameter starts with multiple backslashes like \\\\\\example.com, browser will redirect user to http://example.com.