Metrics: Rework categories for gitdm

GitDM (Git Data Miner) is the tool we use to generate metrics on git commits see (https://github.com/MariaDB/metrics).

At the moment, categories of organisations are calculated using a clone of the config with a search/replace on the organisations and re-running. This is flawed in a number of ways. In particular, keeping this up to date.

Instead we need to have a configuration file that maps organisations to categories, this is simpler to maintain, more accurate and output would be generated at the same time as everything else.