Set UTF-8 for Wolfplex forms encoding

Summary:
https://www.wolfplex.org/forms/ contains UTF-8 forms.
Some browsers still try to read those as ISO-8859-1 by default.

Test Plan: Patched live in July 2023 (according stat).

Reviewers: dereckson, ledesillusionniste

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3517