Notify Mattermost when issues labeled as ~Crash are created

Let's say we want every new issue labeled with Crash in lhcb/Moore be posted on the Upgrade HLT2 channel.

We can follow the recipe in https://mattermost.gitbook.io/plugin-gitlab/setup/configuration#step-4-subscribe-to-projects-and-groups to achieve that. You need to be a Maintainer/Owner of the lhcb/Moore project.

Open the channel in Mattermost and type

/gitlab connect
/gitlab webhook add lhcb/Moore IssuesEvents,SSLverification
/gitlab webhook list lhcb/Moore
/gitlab subscriptions add lhcb/Moore issues,label:"Crash"
/gitlab subscriptions list

Related to !1252 (merged)