Use upstream slack-go/slack again (#1018)

vendor/github.com/slack-go/slack/comment.go

@@ -0,0 +1,10 @@
+package slack
+
+// Comment contains all the information relative to a comment
+type Comment struct {
+	ID        string   `json:"id,omitempty"`
+	Created   JSONTime `json:"created,omitempty"`
+	Timestamp JSONTime `json:"timestamp,omitempty"`
+	User      string   `json:"user,omitempty"`
+	Comment   string   `json:"comment,omitempty"`
+}