Add foreign key on event in addition to blockNumber

When I started the indexer, blockNumber was enough for my needs, since I then switched to RPC. But with the kind of usage in Cesium v2, we would need to get more info on the block details:

these clickable links goes to the block

See forum discussion: https://forum.duniter.org/t/duniter-squid/11858/12

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So we could replace most blockNumber Int fields by a foreign key on the block.