(bug 44424) wikiversions.cdb for labs

This patch adapts multiversion to detect an eventual
wikiversions-labs.cdb file whenever it exists.  With git-deploy we will
have to add the .cdb files to git in order to deploy them, so we want
wikiversions-labs.dat to generate a different cdb file.

Reinstate https://gerrit.wikimedia.org/r/#/c/46240/
Commit f2e28f69b

Change-Id: I77e400c97840d9b6671bbbe34e0329fbf0397762
Merge "(bug 44424) wikiversions.cdb for labs"