[US] As a user, I want to have a multichain Explorer, regardless of the networks where Verax is deployed

[alainncls]

Note

This needs to be merged onto the feat/multichain branch

Current behaviour

Today, the Explorer can only display data for one given network at a time.
Technically speaking, the SDK is instantiated to target one single network, and the URLs when navigating contain the network's name.

Tasks to cover

• Upgrade to SDK V3 ("Multichain mode")
• Instantiate the SDK in only 2 modes ("Testnet" to get all testnet data, "Mainnet" to get all mainnet data)
• Stop considering the network name when reading the URLs path
• Remove the network name when creating the URLs
• Remove the networks dropdown selector from the header navbar
• Add a toggle selector in the header navbar to switch between testnets and mainnets
• Mainnets are selected by default