[Cashtab Extension] Close open pop-ups before opening a new one

Summary:
Depends on D14692

T3312

Before opening an extension pop-up window, close any existing extension popup windows

This prevents a user issue experienced now that PayButton is live, where a user may wish to complete multiple transactions, and end up with multiple identical small windows open.

Test Plan:
npm run extension
load unpacked extension into brave or chrome
visit components.cashtab.com
Click on different Cashtab Badge buttons and observe that you do not have more than one popup open at any given time

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14696