Remove slug and move to static file with chain id -> coin id mapping

[bernatfp]

As part of this, the metadata file should use chain id instead of slug. This way we avoid introducing a new field in the config.

Should we also migrate from Coinmarketcap to Coingecko?

[bernatfp]

10K call credits on coingecko should be enough since we don't need to call often. Let's do it!