Fix GiftAidOverviewController

Problem/Motivation

GiftAidOverviewController is a stub. I imagined it as an important piece of UI. For example, for a user, it would form the basis of what was presented under a 'Gift Aid' tab.

We need to:
(a) figure out how it should it work, for donors and for staff
(b) figure out what's core and what's specific to the context so belongs in e.g. gift_aid_user
(c) fix it

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes