Increase upload limit to 100M for wikis (1/2 - nginx)

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.

Default value for nginx is much lower.

See also D3951 for PHP part.

Test Plan: Upload a large file

Reviewers: Duranzed, dereckson

Reviewed By: Duranzed

Differential Revision: https://devcentral.nasqueron.org/D3950