perf(storage): pair all SELECT true with LIMIT 1

Apparently, postgresql isn't smart enough to realize that once a true value
value is found as part of a `SELECT true`, there is no need to scan the rest of
the table, so we have to make this explicit. We could also have used the
`SELECT EXISTS(…)` construct, but it's more verbose and I don't like it.

internal/storage/icon.go

@@ -15,7 +15,7 @@ import (
 // HasFeedIcon checks if the given feed has an icon.
 func (s *Storage) HasFeedIcon(feedID int64) bool {
 	var result bool
-	query := `SELECT true FROM feed_icons WHERE feed_id=$1`
+	query := `SELECT true FROM feed_icons WHERE feed_id=$1 LIMIT 1`
 	s.db.QueryRow(query, feedID).Scan(&result)
 	return result
 }