[Cashtab] New token URL validation

Summary: Altered the isValidTokenDocumentUrl function to include a simple regex url valiation pattern, updated validation.test.js to include new unit tests, and updated warning message to reflect new conditions for token URLs.

Test Plan:
cd web/cashtab
npm start
navigate to the createToken form
begin inputting url
ensure that if the conditions aren't met, an error message will be displayed
ensure that when the conditions are met, the error message disappears
ensure error message is adequate in CreateTokenForm.js

in a new terminal:
npm test
ensure all tests pass

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D10650