Post-merge reminder

I noticed that I often need to do some action after some MR is merged. It would be cool to be able to add special markdown block to a description that will be sent to my email after this particular MR is merged. Something like

[post-merge]: [@user1 @user2] Notify a customer that this is already merged

Possible use cases:

• Update some customers that are requested this feature or bugfix but they are not mentioned in this MR.
• Remind to create some issue after merging (for me it was the case several times)
• Prepare production servers for future deploy
• Something else...

/cc @JobV @victorwu @smcgivern