wger: trainer_login accepts GET - CSRF bypass enables forced session rebinding

wger is a free, open-source workout and fitness manager. Prior to version 2.6, the `trainer_login` view in wger accepts GET requests and executes `django_login()` without any CSRF protection, because Django's `CsrfViewMiddleware` only enforces tokens on unsafe methods (POST/PUT/PATCH/DELETE). An attacker can embed a single `<img>` tag on a malicious page; when an authenticated trainer loads that page, their browser auto-issues the GET with the session cookie, forcibly rebinding the trainer's session to an arbitrary user account. Version 2.6 fixes the issue.