search: move alert observer out of graphqlbackend package

Created by: rvantonder

Something is wrong with main-dry-run CI setup. Please review, I will retry/get it green.

As in title: this moves remaining alert logic (in alert observer) out of graphqlbackend. I put the alert observer stuff in it's own internal/search/alert as observer.go, because it relies on a lot more state that causes circular dependencies at the internal/search level. There may be ways to unify things later, but re: just getting this out of graphqlbackend:

Some inline comments.