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

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

@@ -0,0 +1,26 @@
+package slack
+
+// ActionBlock defines data that is used to hold interactive elements.
+//
+// More Information: https://api.slack.com/reference/messaging/blocks#actions
+type ActionBlock struct {
+	Type     MessageBlockType `json:"type"`
+	BlockID  string           `json:"block_id,omitempty"`
+	Elements BlockElements    `json:"elements"`
+}
+
+// BlockType returns the type of the block
+func (s ActionBlock) BlockType() MessageBlockType {
+	return s.Type
+}
+
+// NewActionBlock returns a new instance of an Action Block
+func NewActionBlock(blockID string, elements ...BlockElement) *ActionBlock {
+	return &ActionBlock{
+		Type:    MBTAction,
+		BlockID: blockID,
+		Elements: BlockElements{
+			ElementSet: elements,
+		},
+	}
+}