Enable subpages on wolfplex Bulletin: namespace

Summary:
[[Bulletin:Main]] offers {{/{{#time: Y/m}}}}.

The expected behavior it's to include [[Bulletin:Main/2018/03]].

The actual behavior is the page tries to include [[Template:/2018/03]].

The expected behavior only works if subpages are enabled for the namespace.

Test Plan: Purge https://www.wolfplex.org/wiki/Bulletin:Main

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1503