Rename some constants to clarify their meaning and purpose

* MULTIVER_COMMON_APACHE => MEDIAWIKI_DEPLOYMENT_DIR.
* MULTIVER_COMMON_HOME => MEDIAWIKI_STAGING_DIR.
* Also delete MULTIVER_404SCRIPT_PATH_HOME & MULTIVER_404SCRIPT_PATH_APACHE.
* Renamed activeMWVersions.php's '--home' arg to '--staging', but added code to
  support the old name.

Change-Id: I4e41872854e9e8c536b4d353f4e3f4b55313278d