Make`Chain Info` link configurable

[traumschule]

Add config entry for Chain Info sidebar link

There are currently multiple deployments available:

• https://apps.joyutils.org/?rpc=wss://rpc.joyutils.org#/staking/actions
• https://blocks.l1.media/?rpc=wss://rpc.l1.media#/legacy-staking
• http://blocks.joystream.dev/?rpc=wss://rpc.joystream.dev#/explorer

See also Joystream/handbook#49