Track liveness of validator nodes/entities

[csillag]

First, a non-coding task: Figure out how liveness is determined/queried.

Then we'll add tracking of liveness (including history!) in our DB.

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The validator pages on the Explorer mocks include a validator 'uptime' and a 'participation rate' which refer to the same metric. Add that metric to the Validator object.

Currently, we define this metric as (# of blocks signed/proposed by a validator over prior 24hrs / # total blocks produced over prior 24hrs). The length of the time frame is still open to debate, though it'd be preferable to use the same time frame that etherscan/oasisscan use.

Once #646 is merged, the consensus analyzer will store the block signer/proposer data in the db. We'll need to check whether directly querying the blocks table is performant enough, otherwise we may need to preprocess the signer/proposer info in a materialized view or something.

[ptrus]

The remaining part will be implemented in: #800