[cashtab] Don't open agora offers on a new tab

Summary:
This is the same website, and should not open in a new tab. This makes the mobile experience counter-intuitive.

Ref T3715

Test Plan: Run cashtab and check it opens in the same tab

Reviewers: #bitcoin_abc, emack, bytesofman

Reviewed By: #bitcoin_abc, emack, bytesofman

Subscribers: emack, bytesofman

Maniphest Tasks: T3715

Differential Revision: https://reviews.bitcoinabc.org/D17445