Feature Request: Configure who to assign if merge doesn't work

[georgkrause]

Currently Marge always assigns the creator of the Merge Request, however it might be better to assign the person who told Marge to take care for the Merge Request again (or maybe someone else).

We are an open source project and have a lot of merge requests created by our users. They don't really care for all the obligations that need to be met to make the CI happy. If I as a maintainer decide to assign Merge to get it merged and something goes wrong, I need to be assigned again in order to take care for whatever is needed.

An alternative would be to mention me on the error comment marge does, is this somehow configurable?

[georgkrause]

I did some investigations and the Gitlab API doesn't easily provide the information who assigned the PR to Marge, so its not easy to use this to assign someone. Anyways, having a configuration to specify who is responsible for merging if Marge cannot do it would be really helpful!