[Cashtab] Enable special features if wallet has specific eToken

Summary:
Add a method to check the wallet for an arbitrary token. Conditionally render react component if token balance found. This diff includes one feature inspired by https://dogechain.info/

This can be extended for other token-enabled features e.g. dark mode, token minting, token icons.

Test Plan:
npm test to verify new method

Ping me in slack with your eToken address and I'll send you some tabcash to enable testing this specific feature.

Verify feature is enabled for wallet with a balance of the tabcash eToken. Verify feature is disabled for wallets without a balance in tabcash.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9250