[Cashtab] Instructions for adding icon to pre-existing token

Summary: Added instructions for token icon creation for already created tokens. Related to task T1982.

Test Plan:
cd web/casthab
npm start
navigate to token page, ensure alert is present and that copy is sufficient.
navigate to send token page, ensure alert is present and that copy is sufficient.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10533