Increase upload limit to 100M for wikis (2/2 - PHP)

Summary:
MediaWiki allows by default uploads up to 100 MB. This makes sense nowadays when
trying to upload larger files, like high-resolution pictures from a smartphone.

See also D3950 for nginx part.

Test Plan: Upload a large file to Agora

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3951