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

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

@@ -0,0 +1,32 @@
+package slack
+
+// ContextBlock defines data that is used to display message context, which can
+// include both images and text.
+//
+// More Information: https://api.slack.com/reference/messaging/blocks#actions
+type ContextBlock struct {
+	Type            MessageBlockType `json:"type"`
+	BlockID         string           `json:"block_id,omitempty"`
+	ContextElements ContextElements  `json:"elements"`
+}
+
+// BlockType returns the type of the block
+func (s ContextBlock) BlockType() MessageBlockType {
+	return s.Type
+}
+
+type ContextElements struct {
+	Elements []MixedElement
+}
+
+// NewContextBlock returns a new instance of a context block
+func NewContextBlock(blockID string, mixedElements ...MixedElement) *ContextBlock {
+	elements := ContextElements{
+		Elements: mixedElements,
+	}
+	return &ContextBlock{
+		Type:            MBTContext,
+		BlockID:         blockID,
+		ContextElements: elements,
+	}
+}