FreeScout's Mailbox OAuth disconnect uses a state-changing GET and is CSRFable

FreeScout is a free self-hosted help desk and shared mailbox. Prior to version 1.8.215, the mailbox OAuth disconnect action is implemented as `GET /mailbox/oauth-disconnect/{id}/{in_out}/{provider}`. It removes stored OAuth metadata from the mailbox and then redirects. Because it is a GET route, no CSRF token is required and the action can be triggered cross-site against a logged-in mailbox admin. Version 1.8.215 fixes the vulnerability.