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

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

@@ -0,0 +1,22 @@
+package slack
+
+// DividerBlock for displaying a divider line between blocks (similar to <hr> tag in html)
+//
+// More Information: https://api.slack.com/reference/messaging/blocks#divider
+type DividerBlock struct {
+	Type    MessageBlockType `json:"type"`
+	BlockID string           `json:"block_id,omitempty"`
+}
+
+// BlockType returns the type of the block
+func (s DividerBlock) BlockType() MessageBlockType {
+	return s.Type
+}
+
+// NewDividerBlock returns a new instance of a divider block
+func NewDividerBlock() *DividerBlock {
+	return &DividerBlock{
+		Type: MBTDivider,
+	}
+
+}