Fixes#669.
Composes all tx components to create a send transaction modal for the wallet.
1. Add a reusable TransactionStackView component that wraps a StackView component to show the screens of the Send Tx modal and handles back/forward/reset functionality.
2. Add a reusable TransactionStackGroup which holds tx components and allows modal header and button text to be defined and handles validation for all child components.
3. Add an isValid property to all tx comps for pristine validation state.
4. Reset all components in modal once modal is closed. This consists of a `resetInternal` function that each component should implement to reinstate its original starting state, and a `reset` function that parent components can use to reinstate the overridden properties.
5. Tx error handling to display either a StatusGo error message in the dialog, or wrong password in the TransactionSigner.
6. Fix ReceiveModal to allow for pre-selected account based on current wallet account.
7. Add focused border colour to Input component.
8. Fix issue with last TransactionStackGroup input not being able to obtain focus.
9. Fix fiatBalance not appearing on initial load on AccountSelector.
10. Fix selected asset updated properly when assets changed in the AssetSelector component.
11. AccountSelector is pre-populated with selected wallet account. Supporting work on the components has been done to suppor this.
12. Changing accounts in the "from account" updates the asset balances in the AssetAndAmountInput component.
13. Move validation from ContactSelector to the Select component.
1. Test sending of tokens. This requires that tokens contracts are setup on testnet. Right now, they are set up for contract addresses on mainnet.
2. Loading state once transaction is sent. Button in modal needs to move to a loading state and the "toast" messages in the wallet need to appear informaing user of tx progress.
3. Need to clarify (and implement?) support of ENS names in the AddressInput. It appears that ENS names could be resolved. This would be a long operation and require some kind of UI loading indication.
4. Wallet balances need to be updated on every block, so for example, sending funds between accounts I should be able to see the balance updated in real time.
1. Sending to a contact currently doesn't work because the ContactSelector component selects the Contact's whipser key, instead of his/her wallet address. May need to figure out how this is done in status-react. As it stands, attempting to send to a contact will crash the app.
2. Sending *from* an imported account does not work, with an error from StatusGo `cannot locate account for address: 0x123...`
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