[MM-16798] Add a Guest Accounts feature flag (#13231)

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

api4/team.go

@@ -968,7 +968,7 @@ func inviteUsersToTeam(c *Context, w http.ResponseWriter, r *http.Request) {
 }
 
 func inviteGuestsToChannels(c *Context, w http.ResponseWriter, r *http.Request) {
-	if c.App.License() == nil {
+	if c.App.License() == nil || !*c.App.License().Features.GuestAccounts {
 		c.Err = model.NewAppError("Api4.InviteGuestsToChannels", "api.team.invate_guests_to_channels.license.error", nil, "", http.StatusNotImplemented)
 		return
 	}