UX improvements

Various UX improvements:

• Displays Keeper download link when Keeper is not found
• Secrets now must be created (with repeated input) before being used
• Error handling for node interaction functions (sending payments, etc.); retry button appears on errors
• Recipient address is also displayed after entering the recipient pubkey
• Warning on leaving the contract creation window between contract setup and payment; warning on deleting the contract
• Contract state is updated properly every 2 seconds
• Entering an invalid address (no contract at address or not a valid address at all) into the recipient's address input shows an error message
• The app now works on both testnet and mainnet

Refactors:

• All blockchain interactions (Keeper and node) are abstracted in a single "Blockchain Connector" entity
• Asynchronous functions return promises instead of accepting callbacks
• Components are separated into different files