add new Mantle extension, required by coming Flow

Mention it in extension-list and require it if $wmgUseFlow.
Remove the similar changes to the -labs versions of these files that
were made earlier to deploy the extension only on beta labs.

Bug: 66094

Change-Id: Ifaacd9843a7e6e344734a07eec988474dc8b3ba5
Merge "add new Mantle extension, required by coming Flow"