[Cashtab] Added eToken icon to send page

Summary:
Using the same logic as TokenListItem, eToken icon added to send page. Related to task T1988.

Custom Token:

Generated Token:

Test Plan:
cd web/cashtab
npm start
navigate to eTokens tab
select a token with a custom icon
ensure icon displays at top of token table
navigate back to eTokens tab
select a token with a generated icon
ensure icon displays at top of token table

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D10649