CN/TrEx 3 -- Message Group Review & CN Namespace

Similar to the abandoned https://gerrit.wikimedia.org/r/#/c/24638/
which had to be abandoned because git review is dumb...

Using the message group created in CN/TrEx 2; we now apply translation
review features to CentralNotice messages. This is done by creating
messages in a new CentralNotice namespace (allowing everyone to edit
banner text) and then moving the messages to the MW namespace when
the review is changed to the correct state.

States are defined in https://gerrit.wikimedia.org/r/#/c/26769/

Change-Id: I73c5716807567aecce19584bf34744f5097edf1b