Define which parts of Pombola are core/country-specific/obsolete

[jacksonj04]

• [x] Create a new branch in the existing repository (#2604)
• [x] Scrub away anything which isn't involved in making either Kenya or South Africa work (with some extra judgement calls made for Nigeria)
• [x] Make notes on where country-specific code appears in unexpected places (ie anywhere which is not in a country-specific directory)
• [x] Fix where possible anything where non-country-specific code is summoned from a country-specific directory. In theory this shouldn't happen, but you never know.

[chrismytton]

This is now done. Tracking the actual repo split in https://github.com/mysociety/pombola/issues/2630.