[Cashtab] Move chronik out of utils

Summary:
Move chronik functions, unit tests, and mocks out of utils and into new dir at src/chronik

Update imports for new location

Test Plan:
npm test
grep -r utils/chronik src/ and no output

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15253