[CI] Restore cashtab preview

Summary: Recent changes in the nginx configuration prevent cashtab from being rendered on preview. This diff restores the preview by creating a simplified server config file that is used to build the preview.

Test Plan:

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

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15183