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

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

@@ -0,0 +1,73 @@
+package slack
+
+// @NOTE: Blocks are in beta and subject to change.
+
+// More Information: https://api.slack.com/block-kit
+
+// MessageBlockType defines a named string type to define each block type
+// as a constant for use within the package.
+type MessageBlockType string
+
+const (
+	MBTSection MessageBlockType = "section"
+	MBTDivider MessageBlockType = "divider"
+	MBTImage   MessageBlockType = "image"
+	MBTAction  MessageBlockType = "actions"
+	MBTContext MessageBlockType = "context"
+	MBTInput   MessageBlockType = "input"
+)
+
+// Block defines an interface all block types should implement
+// to ensure consistency between blocks.
+type Block interface {
+	BlockType() MessageBlockType
+}
+
+// Blocks is a convenience struct defined to allow dynamic unmarshalling of
+// the "blocks" value in Slack's JSON response, which varies depending on block type
+type Blocks struct {
+	BlockSet []Block `json:"blocks,omitempty"`
+}
+
+// BlockAction is the action callback sent when a block is interacted with
+type BlockAction struct {
+	ActionID             string              `json:"action_id"`
+	BlockID              string              `json:"block_id"`
+	Type                 actionType          `json:"type"`
+	Text                 TextBlockObject     `json:"text"`
+	Value                string              `json:"value"`
+	ActionTs             string              `json:"action_ts"`
+	SelectedOption       OptionBlockObject   `json:"selected_option"`
+	SelectedOptions      []OptionBlockObject `json:"selected_options"`
+	SelectedUser         string              `json:"selected_user"`
+	SelectedChannel      string              `json:"selected_channel"`
+	SelectedConversation string              `json:"selected_conversation"`
+	SelectedDate         string              `json:"selected_date"`
+	InitialOption        OptionBlockObject   `json:"initial_option"`
+	InitialUser          string              `json:"initial_user"`
+	InitialChannel       string              `json:"initial_channel"`
+	InitialConversation  string              `json:"initial_conversation"`
+	InitialDate          string              `json:"initial_date"`
+}
+
+// actionType returns the type of the action
+func (b BlockAction) actionType() actionType {
+	return b.Type
+}
+
+// NewBlockMessage creates a new Message that contains one or more blocks to be displayed
+func NewBlockMessage(blocks ...Block) Message {
+	return Message{
+		Msg: Msg{
+			Blocks: Blocks{
+				BlockSet: blocks,
+			},
+		},
+	}
+}
+
+// AddBlockMessage appends a block to the end of the existing list of blocks
+func AddBlockMessage(message Message, newBlk Block) Message {
+	message.Msg.Blocks.BlockSet = append(message.Msg.Blocks.BlockSet, newBlk)
+	return message
+}