[ticket/11566] add interface for captcha

Add basic captcha template while reporting post
when the user is not a registered user.

PHPBB3-11566
[ticket/11566] add error functionality

add $error which stores the captcha error when captcha validation fails

PHPBB3-11566
[ticket/11566] add error in template

Check if error exists, then display it
in the report post template.

PHPBB3-11566
[ticket/11566] display error instead of trigger_error

When the error report is empty display error
in the template instead of trigger_error

PHPBB3-11566
[ticket/11566] add captcha reset and hidden fields

If captcha is solved and some other error pops up, store the
captcha in a hidden field.
Reset captcha if reporting the post is successful

PHPBB3-11566
[ticket/11566] add tests for reporting post

Functional test for reporting post and check if captcha validation
is required for guests and not for registerted users

PHPBB3-11566
[ticket/11566] Use language variable instead of hardcode

Add language variable in tests

PHPBB3-11566
[ticket/11566] Revert forum permission changes

Revert the f_report permission for guests in the functional tests

PHPBB3-11566