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

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

@@ -0,0 +1,29 @@
+package slack
+
+import (
+	"context"
+	"net/http"
+)
+
+type WebhookMessage struct {
+	Username        string       `json:"username,omitempty"`
+	IconEmoji       string       `json:"icon_emoji,omitempty"`
+	IconURL         string       `json:"icon_url,omitempty"`
+	Channel         string       `json:"channel,omitempty"`
+	ThreadTimestamp string       `json:"thread_ts,omitempty"`
+	Text            string       `json:"text,omitempty"`
+	Attachments     []Attachment `json:"attachments,omitempty"`
+	Parse           string       `json:"parse,omitempty"`
+}
+
+func PostWebhook(url string, msg *WebhookMessage) error {
+	return PostWebhookCustomHTTPContext(context.Background(), url, http.DefaultClient, msg)
+}
+
+func PostWebhookContext(ctx context.Context, url string, msg *WebhookMessage) error {
+	return PostWebhookCustomHTTPContext(ctx, url, http.DefaultClient, msg)
+}
+
+func PostWebhookCustomHTTP(url string, httpClient *http.Client, msg *WebhookMessage) error {
+	return PostWebhookCustomHTTPContext(context.Background(), url, httpClient, msg)
+}