Captcha form field can be cached by Varnish, which makes submitting the form impossible for the first time (after submitting the image is refreshed).

Steps to reproduce:
0. Setup eZ Platform with Varnish
1. Login as admin
2. Create new Form under Home
3. Add Captcha form field
4. Publish the form
5. Visit the form on the frontend
6. Refresh the page multiple times to make sure it's cached by Varnish
7. Using another browser: visit yoursite/login and login as admin
8. Visit the same form (still using the second browser)

Expected:
Different captcha is displayed

Actual:
The same captcha is displayed.