[Create Account] Fund account with Friend button behavior

[janewang]

What problem does your feature solve?

On the Create Account screen:

• If a user creates an account, the [Fund account with Friendbot] button should fund the account on the screen
• If a user has not created an account, the [Fund account with Friendbot] button should not be clickable and be in a disabled state
• If a user is on mainnet, this button should not be available
• If a user is on a custom work, this button should not be available unless they have inputted Horizon endpint
• (added by Jeesun) clear the the generated public address wasn't cleared when switching network (thanks iveta)