Cologne Blue: add missing 'undeletethispage' message

The link was added in I9486649e (previously the bottom menu only
included a 'Delete this page' link), but the message was never created.

Bug: 48939
Change-Id: I8efaf9c14f918e7e1ac3d68eac7f1e662dfe0204
Merge "Cologne Blue: add missing 'undeletethispage' message"