fix: Prevent error when using quotation marks in BuildMessage

When a PR title or commit summary contains quotation marks, they were
not escaped when injected into the JSON attachment. This caused
a message like

    unable to unmarshal webhook message: invalid character 't' after object key:value pair

when used inside of a template containing this test:

```json
{
  "text": "*Status*: FAILURE\n*Repo*: {{ .RepositoryName }} | *Branch*: {{ .BuildBranch }} | *Author*: {{ .BuildAuthor }}\n*Build*: {{ .BuildLink }}\n*Message*: {{ .BuildMessage }}"
}
```

This change safens this use case. Other typical fields _shouldn't_ have
quotation marks in them, but it might be prudent down the line to
cleanse _all_ fields as they are injected into the JSON or provide
a method usable inside of the template that escapes correctly.

cmd/vela-slack/plugin.go

@@ -91,7 +91,7 @@ func (p *Plugin) Exec() error {
 	// clean up newlines that could invalidate JSON
 	// BuildMessage is the only field that can have newlines;
 	// typically when the commit contains a title and body message
-	p.Env.BuildMessage = strings.Replace(p.Env.BuildMessage, "\n", "\\n", -1)
+	p.Env.BuildMessage = cleanBuildMessage(p.Env.BuildMessage)
 
 	// create message struct file Slack
 	msg := slack.WebhookMessage{
@@ -187,6 +187,13 @@ func (p *Plugin) Exec() error {
 	return nil
 }
 
+func cleanBuildMessage(buildMessage string) string {
+	subject := buildMessage
+	subject = strings.Replace(subject, "\n", "\\n", -1)
+	subject = strings.Replace(subject, "\"", "\\\"", -1)
+	return subject
+}
+
 // Validate function to validate plugin configuration.
 func (p *Plugin) Validate() error {
 	logrus.Debug("validating plugin configuration")

cmd/vela-slack/plugin_test.go

@@ -328,6 +328,29 @@ Newlines`,
 	}
 }
 
+func TestSlack_Plugin_Exec_Quote(t *testing.T) {
+	// setup types
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprintln(w, "ok")
+	}))
+	defer ts.Close()
+
+	p := &Plugin{
+		Webhook: ts.URL,
+		Env: &Env{
+			BuildMessage: `This message has "quotes"`,
+		},
+		Path:       "testdata/slack_attachment.json",
+		WebhookMsg: &slack.WebhookMessage{},
+		Remote:     false,
+	}
+
+	err := p.Exec()
+	if err != nil {
+		t.Errorf("Exec returned err: %v", err)
+	}
+}
+
 func TestSlack_Plugin_Exec_Newline_Embedded(t *testing.T) {
 	// setup types
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {