gerrit-jj par Jakub Bielecki

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

• You did your jj gerrit upload
• Later on, when on Gerrit, you'd want to switch back to jj.
• So, you'd want to jj edit <what>
• The plugin injects the <what> on Gerrit view so it's there in plain sight.
• It's an html modification, the underlying jj message is not touched actually :)

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.