Implement logic that adds a label when feedback from the maintainer is necessary, and removes it otherwise

This includes a bunch of refactoring to make it possible. Things are split in different commits, but the main logic of the MR is the following:

• Make is possible to have web hooks that are not just MergeRequest Events. This includes quite some refactoring and working around. Generics help, but not everything can be done with generics, unfortunately.
• Add a label when assigning a maintainer to an MR. This is set as an option. My only missing wonder is if this should be allowed to be empty, it should probably not crash. I think that's the case right now, but I'm not 100% sure
• Add support for the AutoLabeler, to remove or re-add the same label depending on the maintainer or other's actions. The logic is explained in detail, and there is a TODO that needs discussion
• Add a "MaintainerFeedback" service, that listens for MergeComments webhook. I guess this needs to be further configured, but I am hoping that the logic is correct.