[Core] New optional/ignore message keys

https://gerrit.wikimedia.org/r/#/c/132423/2/languages/i18n/en.json,unified
https://gerrit.wikimedia.org/r/#/c/111096/12/languages/i18n/en.json,unified

Change-Id: I3a16883e361540ca259801d7bc7345f724442f05