deploy_promote.py: Attribute wikiversions.json updates to the initiating user
Add a Initiated by <user>@<host> review comment when +2'ing the
wikiversions.json change.
Mention the change URL in the waiting message.
Refactored some gerrit operations from backport.py into a new
GerritSSH class in gerrit.py. Use GerritSSH in
deploy_promote.sh to push and +2 the wikiversions.json change.
Bug: T401779