Add notification option for raids

[AllieQpzm]

It would be useful for an option to give an alert of some kind when there's a raid for a channel. I tried to accomplish this with highlights, but it appears that system messages don't trigger highlights

[pajlada]

A precursor to this would be to make highlights that are active in certain channels only, once that's implemented I think this would make sense as an addition in Chatterino