Refactor report logic

[chrismytton]

First step of removing report logic from the template so that it's easier to understand/reason about/work with.

Possible next steps:

• Add a class that munges the stats data into a shape that's easier to work with in the template (or possible do that in gender-balance-country-stats.
• Add some tests for the report logic
• Reduce the duplication in the reporting classes

[chrismytton]

Going to merge this and continue with improvements in futures PRs.

[tmtmtmtm]

I thought the whole point of rolling the stats up in another repo/process was to avoid there needing to be any logic here?