[Cashtab] Add a component to handle external links

Summary:
Cashtab links to tokenDocumentUrls which are un-reviewed and could go anywhere. We should notify users.

Introduce a portable component to handle this on the Token page. This component can be used in other places where we may encounter links, like Cashtab Msgs or Airdrop msgs.

Some style modifications to the Modal were necessary to support its use anywhere (being wrapped by the flex table holding the link modified styles)

Test Plan:
npm test

can try it out at cashtab.io, go to a token page with a url e.g. https://cashtab.io/#/token/9e0a9d4720782cf661beaea6c5513f1972e0f3b1541ba4c83f4c87ef65f843dc , click the url, see modal

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D18196