Removed all old accounting and payment related tables ...

Transactions will now be on a per-question ( or per-stab ) basis.
Only schools could have needed a detailed invoice. But we aren't
targeting them now.

Moreover, we could explore in-app payments as an option. And hence,
payment via the web-site is a secondary option.
No need to have a separate behaviour table ...

Student transactions are enough evidence of how students are behaving.
Scaffolding for Gredits
Defined behaviour for stab/ping

This is the only URL called - with different arguments - when
either the Options button the mobile app is clicked or when
one of the menu options is clicked.
Added method to charge student N gredits
Apply charges to student as part of stab/ping