[Cashtab] Move validation out of utils

Summary:
As part of ongoing effort to improve organization of functions in Cashtab, move validation out of the utils/ directory and into its own directory.

Update imports, move mocks, delete legacy mocks files in utils/

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15201