This allows users who upgrade Gerrit to make use of
experimental features or temporarily revert to previous
behavior in case the new behavior breaks them.
The specific use case we have in mind for this is to
turn off patch-set-level comments in 3.3 which breaks
some CI workflows.
Change-Id: Ib1f79cf4c799b0b52e2597801ccff4cbcbfa44c6
be98d8c640