Use mediawiki install from debian instead of deploying ourselves

[frankduncan]

Currently the mediawiki tar is downloaded for each competition. Then it's updated and configured. That means that when mediawiki updates come, we have to update and redeploy all previous competitions fully, which is unlikely to be done. It also doesn't work well with the way we like to keep our systems up to date.

The catch is that MediaWiki is really set up to have on wiki per machine. That means that there's one LocalSettings file for the machine, and one set of extensions. So some magic needs to happen so that the LocalSettings is different for different competitions based on paths.

Likely there will be one installation, with all the extensions that all competitions need in the extensions folder. But then each local settings that's loaded via symlinking magic will load the extensions pertinent to that competition.

There's an issue that running maintenance scripts from the global directory will fail, so on debian updates, if update.php is run, that will fail. Even if we make it so it doesn't fail, it will still need to be re-run for all installed competitions.

[frankduncan]

As part of this, the mediawiki installation should get it's own top level ansible script (like mwlib does now), and then each of the extensions should get their own top level scripts. Enable the extensions should stay in the competitions. This means as extensions release new versions we need, we can just change them in one place and then redeploy.

[frankduncan]

Related to #59, when doing this, splitting out the LocalSettings configuration to have dependent files that are loaded up is a natural way to configure it for our setup.

[kfogel]

See also related issue #55.