Additional information when viewing Donor entity

Problem/Motivation

ContextOverviewController 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. There is also DeclarationContextOverviewForm which seems related/similar.

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

Proposed resolution

In our updated model, this code relates to the donor entity. When viewing that entity we would like to see:

1. current gift aid status summary
2. buttons for actions (depends on context which are available): declare; cancel (will be done in #3505790: Declaration cancellation); edit => change address or add misc correspondence (staff); send written record (staff)
3. list of all declarations (staff)

Add a gift aid tab for a user (sub-module) that is equivalent to viewing a Donor entity with context equal to the user..

Remaining tasks

User interface changes

API changes

Data model changes