[Cashtab] Add twitter card
[Cashtab] Add twitter card

Summary: Add twitter card so that the Cashtab logo appears when the URL is mentioned in tweets

Test Plan: `npm test` to confirm app functionality is not impacted; will need to test with live URL and Twitter Card validator to confirm active

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9206

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