Add Popups extension

This won't be used in any wikis running 1.23wmf17, but
https://wikitech.wikimedia.org/wiki/How_to_deploy_code#Case_1d:_new_extension
says add it to this branch anyway.

Bug: 61167
Change-Id: I3c91badd1c025c98f073be8bf4c45f8163644550