Implement SweepAccount command

[peterzen]

• Add sweepAccount func to walletapi.go (constructTransaction is a good starting template to use)
• Add bind code to ExportWalletAPI
• Implement frontend middleware in lorca.ts
• Add redux reducers/actions in src/features/transactions
• Implement GUI as a modal (ConstructTxDialog is a good starting point as SweepAccount has similar inputs. The modal should be accessible from a (to be added) Tools menu, similarly to that on the Staking page (StakingToolsMenu.tsx):
  

API doc: https://github.com/decred/dcrwallet/blob/master/rpc/documentation/api.md#sweepaccount