[Cashtab] Handle unconfirmed genesis tx

Summary:
T1752

Cashtab displays 12/31/1969 for Genesis Date on the SendToken.js page when a token's genesis transaction has not yet confirmed. This is because tokenStats.timestampUnix is returned as null in this case.

Handle this case and display an informational note to let the user know the token was not created in the 60s.

Test Plan:
npm start
Create a token
Click the token tile to send some of your newly created token
Observe Genesis Date reads Just now (Genesis tx confirming) if tx is unconfirmed

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9892