Create explorer page with registered signers

[jpraynaud]

Issue

In order to give recognition to SPOs already running on the mainnet, we need to create a page on the explorer for example that lists the registered SPO for an epoch.

Idea: We could display the version of the signer that is setup for each pool, at each epoch, to highlight the nodes that runs an unstable or stale version.

To do

Note: (watch-out offsets and the meaning of epoch: epoch should be the Cardano epoch without offset for clarity)

Aggregator

• Add a /signers/registered/:epoch route in the aggregator REST API that lists the pool ids and their associated stakes, and the epoch at which they will sign
• Add a default /signers/registered/latest route variant that gives access to registrations at the current epoch

Explorer

• Add a Signers tab in the explorer that shows the registered signers for the current epoch
• Allow browsing to 5 last epochs in the Signers tab
• Format stake in a clean way like 68.15M ₳
• Display the total stakes registered at the epoch and the total number of signers

Extra credits

• Pie chart or histogram of stake distribution?
• Pagination of the pool ids?
• Search of a pool id?