MM-37299 - add feature flag for invite members ab testing (#18016)

* MM-37299 - add the invite users feature flag value for AB testing

* make the ff a string value

* add suggested changes

Co-authored-by: Pablo Velez Vidal <pablo.velez@mattermost.com>

model/feature_flags.go

@@ -38,6 +38,9 @@ type FeatureFlags struct {
 
 	// Enable the Global Header
 	GlobalHeader bool
+
+	// Enable the Invite Members button on the left panel, possible values = ("none", "sticky", "lhs_button", "user_icon")
+	InviteMembersButton string
 }
 
 func (f *FeatureFlags) SetDefaults() {
@@ -52,6 +55,7 @@ func (f *FeatureFlags) SetDefaults() {
 	f.PluginFocalboard = ""
 	f.TimedDND = false
 	f.GlobalHeader = false
+	f.InviteMembersButton = "none"
 }
 
 func (f *FeatureFlags) Plugins() map[string]string {