[Cashtab] Set rewards eligibility to true on countdown expiration

Summary:
Current behavior: button keeps counting down (negative countdown).

After this: user can click button to claim rewards exactly when countdown runs out.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16112