[Flow] Add new optional and ignorable message keys

"Terms of Use" messages are Wikipedia centric and should be moved
to WikimediaMessages.i18n.txt

https://gerrit.wikimedia.org/r/#/c/104703/27/Flow.i18n.php,unified
https://gerrit.wikimedia.org/r/#/c/105873/5/Flow.i18n.php,unified

Change-Id: I4ca4b0f3a87b19157e7ed4fa5fac2ad8cedf7261