Support adding custom ERC20 tokens

[liamaharon]

The ability to add custom tokens is highly requested by the community. I imagine the implementation could look something like this

• List the default supported tokens in settings
• Button to add user defined tokens via contract address, decimals, symbol similar to MEW
• Use a default icon if we don't have an official one
• User-defined token state should persist between sessions
• If a user-defined token becomes a default supported token, notify the user and remove it from the user defined tokens list
• Make it clear that user-defined tokens have limited functionality compared to default tokens
  • Won't be available for use in nahmii
  • Won't appear in tx history
  • Will appear in wallet on-chain balance
  • Will be an option when making on-chain transactions