Allow to execute post merge quick actions

When a merge request gets merged we usually need to take additional actions like
relabeling related issues, changing its milestones, assignees etc.

Nowadays we have to look for those related issues and execute any action 'manually'. We could have a new merge request tab or widget that allows quick actions to be executed after a merge request gets merged. Something like:

The backend implementation is simple. It basically consists of creating a note that contains the quick action at the target issue, epic, merge request or commit.

We could later extend this to allow executing quick actions on commits like /tag.