[e.cash] Open Cashtab in a new tab when user clicks Create Wallet

Summary:
We should open Cashtab in a new tab and keep the landing page open when the user clicks "create wallet".

Add a new prop openInNewTab to the custom Button component so that Buttons may open links in a new tab. Implement for the "Create Wallet" button on the landing page above the fold.

Test Plan:
Preview, click "Create Wallet", confirm Cashtab opens in a new tab

Confirm the "get eCash" button navigates without opening a new tab (previous behavior)

Reviewers: #bitcoin_abc, Kronkmeister

Reviewed By: #bitcoin_abc, Kronkmeister

Differential Revision: https://reviews.bitcoinabc.org/D16589