Add tier badges to Growth API pages

Add Free tier badges to Growth-related API pages.

doc/api/broadcast_messages.md

@@ -4,9 +4,7 @@ group: Activation
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
 ---
 
-# Broadcast Messages API
-
-> Introduced in GitLab 8.12.
+# Broadcast Messages API **(FREE SELF)**
 
 Broadcast messages API operates on [broadcast messages](../user/admin_area/broadcast_messages.md).
 

doc/api/invitations.md

@@ -4,7 +4,7 @@ group: Expansion
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
 ---
 
-# Invitations API
+# Invitations API **(FREE)**
 
 Use the Invitations API to send email to users you want to join a group or project, and to list pending
 invitations.