archwiki: Enable Interwiki extension

This extension allows to modify the interwiki table [1], and is shipped
with MediaWiki since version 1.21. [2] It is enabled as a temporary
measure until functionality is merged to the MediaWiki core. [3]

[1] https://www.mediawiki.org/wiki/Manual:Interwiki_table
[2] https://www.mediawiki.org/wiki/Extension:Interwiki
[3] https://phabricator.wikimedia.org/T33951