[Cashtab] Better validation for token document url

Summary:
T3528

Improve validation for token document url

• We do not need to build the regex every time this function is run, define this as a const outside the function
• Return distinct error msgs depending on the validation problem
• We also update the newTokenDefaultUrl, since always utf8-encoding bytes for https:// is not necessary and this passes the validation

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16005