[Cashtab][extn] fix placement of popout button in extension

Summary: Related to T2870. Added flex ctn to cashtab logo in the extension to fix issue with the popout button appearing below the logo & nav indicators.

Test Plan:
cd web/cashtab && npm run extension
wait for extension to build
in your browser's extension manager:
click load unpacked
navigate to /extension and select the /dist dir
open the extension
observe that the popout button is now flexed with the cashtab logo

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12880