beta: switch to /srv/mediawiki

On the beta app servers, I made /srv/mediawiki be a symlink to
/srv/common-local, and on deployment-mediawiki01 I also set made
/srv/mediawiki-staging be a symlink to /srv/scap-stage-dir. These are not
Puppetized yet, but will soon be, for both beta and production.

Change-Id: I963c70aa7884a853bc82f31c0148b9ba68713d0f