Allow PDF uploads to Wolfplex wiki

Summary:
Wolfplex would like to upload PDFs like meeting notes.

Reference: https://www.mediawiki.org/wiki/Manual:Configuring_file_uploads#Configuring_file_types

Test Plan: Upload a document through https://www.wolfplex.org/wiki/Sp%C3%A9cial:T%C3%A9l%C3%A9verser

Reviewers: DorianWinty

Reviewed By: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D3236