Rename MW_COMMON{_SOURCE} vars

* MW_COMMON -> MEDIAWIKI_DEPLOYMENT_DIR
* MW_COMMON_SOURCE -> MEDIAWIKI_STAGING_DIR
* /a/common -> /srv/mediawiki-staging
* Preserve old aliases for back-compat.

Change-Id: Ib573b55c1ec686e3c3588c2e5b06dd998a75bc8a
Merge "Rename MW_COMMON{_SOURCE} vars" into production