Constrain default language list for Dialog Sri Lanka to en, ta, si.

* Currently, end users receive Wikipedia Zero banner for all langs
  when access originates from Dialog Sri Lanka network.
* Carrier would like users to have free access to English, Tamil,
  and Sinhala.
* This change adds the X-CS and X-Carrier headers corresponding to
  Dialog Sri Lanka only when Wikipedia Zero users from the Dialog
  Sri Lanka source IPs access the Zero landing page for English,
  Tamil, or Sinhala localized versions of Wikipedia Zero.

Change-Id: I7ff5607d3e8a1cb1407f862705a115ca4e0ef03c
Merge "Constrain default language list for Dialog Sri Lanka to en, ta, si." into production