add prewritten message A/B test feature flag (#18372)

model/feature_flags.go

@@ -43,6 +43,9 @@ type FeatureFlags struct {
 
 	// Enable different team menu button treatments, possible values = ("none", "by_team_name", "inverted_sidebar_bg_color")
 	AddChannelButton string
+
+	// Enable different treatments for first time users, possible values = ("none", "tour_point", "around_input")
+	PrewrittenMessages string
 }
 
 func (f *FeatureFlags) SetDefaults() {
@@ -59,6 +62,7 @@ func (f *FeatureFlags) SetDefaults() {
 	f.PermalinkPreviews = true
 	f.GlobalHeader = false
 	f.AddChannelButton = "by_team_name"
+	f.PrewrittenMessages = "none"
 }
 
 func (f *FeatureFlags) Plugins() map[string]string {