Add callback actions after something happen (like IFTTT)

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I have to create an MR which, after being merged, I have to do some stuff in Gitaly and apply some changes. In order not to forget that, I have to add some info in the MR description, but that doesn't mean that I will remember to do it.

So, I've been thinking, what if we add some kind of callback mechanism where, for example, when an issue is closed or an MR is merged, you could say 'After this MR is closed open an issue in this project, with this description and assign it to me'.

/cc @jramsay @DouweM @smcgivern @stanhu WDYT?