[cashtab-components] rebrand support

Summary: XEC support for cashtab react components.

Test Plan:

cd web/cashtab-components
npm run storybook

Click button under CashtabBadge (price in fiat) to verify extension opens with XEC transaction of correct amount
Click button under CashtabBadge (price in eToken) to verify that extension DOES NOT open

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9782