Consolidate basic rb-site setup functionality into setup_rbsite().

Review Request #11410 — Created Jan. 26, 2021 and submitted — Latest diff uploaded


Review Board

Reviewers makes use of rb-site's internals for some setup, and
needs to do some initialization to get it working. To avoid having to know much about the internals of this code,
this change introduces a new setup_rbsite() function that sets up the
logging and ConsoleUI.

This will shield from some upcoming changes to the
console initialization.

Tested both rb-site and in Python 2.7 and 3.9.