Buy/Sessions/Shopping Cart update

Buy now uses PDO, disallows duplicates + disables the enroll button
after one is entered, echoes errors and successes visibly. Also cleaned
up the code and commented everything.

Commented out the createuser function that made a table for every new
user. I'd prefer to just update table "cart." I can explain my reasoning
if you'd like.

Shopping cart now shows period number (according to Moodle requirements)
and the remove button now works.