[CI] Add support for cashtab preview

Summary: This will make UI changes easier to review. This spins a temporarly yet fully featured cashtab instance.

Test Plan:

./contrib/teamcity/build-configurations.py preview-cashtab

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D13696