Docs MediaWiki installation upgraded

I've upgraded the wiki from 1.20wmf3 to 1.21wmf5. All Wikimedia managed
extensions were also upgraded to versions that match the MediaWiki version.
None of the non-Wikimedia extensions have been upgraded, as that would
require further testing, and can be done separately from MediaWiki.

This should hopefully alleviate some of the session issues, as the older
version of MediaWiki had some known issues with sessions in memcache.

Please let us know if you run into any issues that may be related to the
upgrade.

- Ryan