Deploy common MediaWiki extensions

Summary:
Those extensions are commonly used on MediaWiki installations
and are pretty standard.

Ref T1367

Test Plan:
Enable them on Wolfplex wiki.

Poem and CategoryTree can be enabled everywhere.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1367

Differential Revision: https://devcentral.nasqueron.org/D1494