[ZeroRatedMobileAccess] Add new optional message keys

https://gerrit.wikimedia.org/r/#/c/48150/1/ZeroRatedMobileAccess.i18n.php,unified

Change-Id: I6e5506a0d2dc1b9bbefb0e97afb1790667020812
Merge "[ZeroRatedMobileAccess] Add new optional message keys"