FEATURE: make trust level for message sending configurable

- add min_trust_to_send_messages site setting (default 1) to allow admins
 to configure when messages can be sent between members

lib/guardian.rb

@@ -250,7 +250,7 @@ class Guardian
     # Can't send message to yourself
     is_not_me?(target) &&
     # Have to be a basic level at least
-    @user.has_trust_level?(TrustLevel[1]) &&
+    @user.has_trust_level?(SiteSetting.min_trust_to_send_messages) &&
     # PMs are enabled
     (SiteSetting.enable_private_messages ||
       @user.username == SiteSetting.site_contact_username ||