Filter out Artifact announcements

[TimJentzsch]

Currently, some posts (e.g. from Wyk) are not restricted to Dota and also show Artifact announcements.
Those should probably be filtered out eventually, especially when we have a seperate bot for Artifact.

[zachkont]

I believe when Artifact is out they won't keep posting on the dota subreddit. If they do, we can address this but for now I kind of like that we get some sneak peaks like those

[zachkont]

Apparently, I am not filtering by subreddit for wyrkhm's posts, so this is more relevant now