Database design documentation

[sebb <se...@gmail.com>]

The database design is fundamental to the operation and code, yet
AFAICT it has never been discussed on the dev list, and I could find
no design documentation.

We can use the experience of the classic version to make the foal
version better.

There are various questions that need to be addressed:
- what fields need to be indexed?
- how are the fields to be derived, do they need to be canonicalised?
- what indexes need to exist, and how are they related?
- what are the primary keys
- etc.

Sebb.