Clarify definition of agendas/tasks etc.

[niss36]

Pasting some parts of what John wrote on the Google doc:

• Tasks are intended to be coherent exercises, such as an exam question or a lecturer’s complete exercise sheet.
• TaskDetails are the individual pieces of a task, such as the parts of an exam question.
• Supervisors create Agendas, which are lists of Tasks that they want their students to attempt for each supervision. This is the equivalent of me telling people to do two past exam questions for a SV.