Fire webhooks for System Notes

Problem to solve

Get comment notification after a commit mentioning specific issue/mergerequest

Further details

Currently it's possible to send webhooks on comments and push. However the information about commit mentioning an issue is displayed "like a comment" on gitlab side, which suggests that it would be great to have the possibility to replicate this information to integrated platforms. Currently it's not possible since push event notification doesn't have info about the issue.

Proposal

Send comment notification when the commit mentioning issue is discovered.