Add beaconChainExplorerUriTemplate property

[iamacook]

Summary

Resolves #1240

Due to restrictions of staking, we link to the Beacon Chain explorer to see the value of specific validators. At the moment, this is done via a client-side hardcoded value.

This adds a new beaconChainExplorerUriTemplate property, similar to blockExplorerUriTemplate. The reason for not adding the value directly is for extensibility as it is also possible to search for other values other than public keys, e.g. block number/hash, graffiti or state hash.

Changes

• Add nullable beacon_chain_explorer_uri_public_key_template property.
• Add relevant serializer and map property
• Add mock value for tests.
• Generate migration and add relevant test.

[coveralls]

Pull Request Test Coverage Report for Build 11102259998

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 99.501%

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Totals
Change from base Build 10955950299:	0.005%
Covered Lines:	1197
Relevant Lines:	1203

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💛 - Coveralls