Perform initial PHP before sending the HTML head

If we want to deposit cookies, we’ll need to do so before sending any
HTML, therefore it’s better to execute the main PHP scripts prior to
sending the HTML <head> stuff.
Deposit cookie to prevent excluded participants

If a user is told they’re ineligible, a cookie is deposited on their
system identifying the experiment ID. If they then try again to sign up
for the experiment again, even with a different email address, they’ll
continue to be told that they’re ineligible. The cookie will expire
after one week.