Add backport wizard

[xmo-odoo]

The mergebot has good support for forward-porting, but from time to time we realise after the fact that a change should have been merged earlier. Currently this requires manually cherrypicking the PR into the earlier branch, then creating the PR, and correctly configuring it.

The mergebot / forwardbot could do that on behalf of the user(s), initially probably via a simple wizard as it's a pretty uncommon need. Eventually this might become a full-blown backport feature à la forwardport except opt-in instead of opt-out.