Add StarterLink as a field to the Github Service config

Use it verbatim in the responses to !commands when the caller has not authed
with Github.

src/github.com/matrix-org/go-neb/matrix/types.go

@@ -1,6 +1,7 @@
 package matrix
 
 import (
+	"encoding/json"
 	"html"
 	"regexp"
 )
@@ -114,3 +115,29 @@ func GetHTMLMessage(msgtype, htmlText string) HTMLMessage {
 		FormattedBody: htmlText,
 	}
 }
+
+// StarterLinkMessage represents a message with a starter_link custom data.
+type StarterLinkMessage struct {
+	Body string
+	Link string
+}
+
+// MarshalJSON converts this message into actual event content JSON.
+func (m StarterLinkMessage) MarshalJSON() ([]byte, error) {
+	var data map[string]string
+
+	if m.Link != "" {
+		data = map[string]string{
+			"org.matrix.neb.starter_link": m.Link,
+		}
+	}
+
+	msg := struct {
+		MsgType string            `json:"msgtype"`
+		Body    string            `json:"body"`
+		Data    map[string]string `json:"data,omitempty"`
+	}{
+		"m.notice", m.Body, data,
+	}
+	return json.Marshal(msg)
+}

src/github.com/matrix-org/go-neb/services/github/github.go

@@ -28,6 +28,7 @@ type githubService struct {
 	ClientUserID       string
 	RealmID            string
 	SecretToken        string
+	StarterLink        string
 	Rooms              map[string]struct { // room_id => {}
 		Repos map[string]struct { // owner/repo => { events: ["push","issue","pull_request"] }
 			Events []string
@@ -49,9 +50,10 @@ func (s *githubService) RoomIDs() []string {
 func (s *githubService) cmdGithubCreate(roomID, userID string, args []string) (interface{}, error) {
 	cli := s.githubClientFor(userID, false)
 	if cli == nil {
-		// TODO: send starter link
-		return &matrix.TextMessage{"m.notice",
-			userID + " : You have not linked your Github account."}, nil
+		return matrix.StarterLinkMessage{
+			Body: "You need to OAuth with Github before you can create issues.",
+			Link: s.StarterLink,
+		}, nil
 	}
 
 	if len(args) < 2 {