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

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

@@ -0,0 +1,28 @@
+package slack
+
+// ImageBlock defines data required to display an image as a block element
+//
+// More Information: https://api.slack.com/reference/messaging/blocks#image
+type ImageBlock struct {
+	Type     MessageBlockType `json:"type"`
+	ImageURL string           `json:"image_url"`
+	AltText  string           `json:"alt_text"`
+	BlockID  string           `json:"block_id,omitempty"`
+	Title    *TextBlockObject `json:"title"`
+}
+
+// BlockType returns the type of the block
+func (s ImageBlock) BlockType() MessageBlockType {
+	return s.Type
+}
+
+// NewImageBlock returns an instance of a new Image Block type
+func NewImageBlock(imageURL, altText, blockID string, title *TextBlockObject) *ImageBlock {
+	return &ImageBlock{
+		Type:     MBTImage,
+		ImageURL: imageURL,
+		AltText:  altText,
+		BlockID:  blockID,
+		Title:    title,
+	}
+}