[Core] Add new message key to ignore

https://gerrit.wikimedia.org/r/#/c/166203/1/languages/i18n/en.json,unified

Change-Id: I3091b86400b65681d4d98a5de38c6887d6ad593a
[WikiEdDiff] Register extension

Change-Id: I538f4147c4ca3e0ace5b870d84377c261f51107d