Captcha form field is optional.

Following common sense captcha field should be required by default, now it's permanently optional.

Steps:
1. Create new LP.
2. Add Form block.
3. Add e.g. single line field.
4. Add captcha field.
5. Submit.
6. Publish.
7. Enter single line field value.
8. Leave captcha field empty.
9. Submit.
Actual: It's possible to enter submission with empty captcha field.
Expected: Captcha field is required by default.

Remarks: To be confirmed with supriya.bhargava@ez.no. (Should the Required checkbox be present?)