Add support for adding custom tokens and listing supported tokens

[navi-arneja]

User Story: As a token holder, I want to add my supported custom tokens to Galleon so that I can view all my token balances in one place.

Implementation: Ensure Ledger users are able to keep state. Consider using either a file wallet or local storage

Designs: https://www.figma.com/file/KyMmhh4XU31uzaPO1oTvwY/Tokens-and-Metadata?node-id=351%3A0