Set an 'openlab-visitor' cookie when visiting the site

This is coupled with a rule in .htaccess that forbids posts to wp-comments-post.php
that do not contain the cookie header. Prevents some form of comment spam bots from
DoSing the system
Ensure that kb-gradebook form requests go to https addresses when appropriate.

Fixes #1161
Merge branch '1.5.x' of github.com:livinglab/openlab into 1.5.x