only push irc issues messages on open/close

Similarly to pull requests (457bab4db8bb70e72b85e7fbe140ae3ce3cde364), only open and close actions for issues should be announced.

After I managed to set the hook's types to "issues" using the API I was confronted with a lot of `... labeled issue #...` spam that nobody cares about. This PR removes that.

I don't know which actions are send overall (couldn't find actions docs, only for types), so I just restricted them to open and close like for pull requsts.