Address discovery on wallet import

[nop33]

This PR:

• Performs address discovery when importing a wallet (while the user sees the welcome confetti screen)
• Outsources to workers the workload of deriving addresses:
  • based on indexes found in local storage when unlocking a wallet
  • when generating 1 address per group
• Cleans up app loading logic
• Introduces the activeWalletSlice store and removes the wallet from the global context
• Cleans up global and addresses contexts

Things that still could be improved:

• The loading animation freezes after the worker has finished. The problem is that our method of updating the state is not optimal (we create a new Map and replace the existing one). This should be improved when switching to Redux with normalized data.