Discourse: ensure forum.yml is ASCII and not UTF-8

Summary:
Upstream isn't willing to set default format as UTF-8 and want to
force ASCII, to avoid to force a specific encoding.

As our file uses non breakable spaces, that makes the container
launcher script fail loudly. We so remove such spaces.

Fixes T815.

Test Plan: iconv config/forum/forum.yml.tmpl -f utf-8 -t ascii

Reviewers: dereckson

Maniphest Tasks: T815

Differential Revision: http://devcentral.nasqueron.org/D368