connect.8x8.com: Blind SSRF via /api/v2/chats/image-check allows for Internal Ports scan

@yassinek3ch reported to us a Blind SSRF in the 8x8 Connect application, in the ChatApps module, which potentially could have allowed for internal port scans. The vulnerable API path was `/api/v2/chats/image-check` via the `url` JSON parameter. Example: ``` POST /api/v2/chats/image-check HTTP/1.1 Host: connect.8x8.com … … Content-Type: application/json Content-Length: 41 {"url":"http://127.0.0.1:/?a=a.png"} ``` We ultimately retired the entire API path, which resolved the issue.