we generally aim to share the load between the two providers equally
(more or less). When one provider has struggled, we deprioritise them,
this commit adds a function that gradually restores balance. It checks
every five minutes, if it's been more than an hour since the providers
were last changed then it adjusts them towards a 50/50 split. Except
it's not quite 50/50 due to #reasons (we want to slightly favour MMG),
it's actually 60/40. That's defined in a new dict in config.py.
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