[Cashtab] Create TokenIcon Component

Summary: Created TokenIcon component and added it throughout the app.

Test Plan:
cd web/cashtab
npm start
Ensure that there are token txs within recent history
Check that custom token icons and generated icons are appearing
Naviagate to eTokens tab
Check that custom token icons and generated icons are appearing
Navigate to SentToken page
Check that custom icons and generated icons are appearing in the amount field of the send form

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11097