[CI] Use a random port for the website preview

Summary:
And write the generated preview URL to both stdout and a log file so it can be later retrieved by the build bot. The IP used is localhost if it's not running on a Teamcity build agent, and the public IP otherwise. To achieve that, the whatismyip package is used.
The port is randomly selected in a range free of known use.

Test Plan:

./contrib/teamcity/build-configurations.py preview-e.cash

Check the preview URL is working as expected.

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13667