gerrit-jj per Jakub Bielecki

Augment the Gerrit change description page with the Change-Id in jj format. This situation:

• you did your jj gerrit upload
• later you look at it on Gerrit, and want to switch back jj to the work
• you'd want to jj edit <what>
• the plugin injects the <what> on Gerrit view so it's there in plain sight.

Look at the screenshots. Nothing else happens. No options or configuration. No storing or sending out any data, neither locally nor remotely.

Tested on Gerrit 3.13.3-982 and 3.12.2-5.