Fix deprecated argument order for "implode()"

Summary: Fixes T13428. In modern PHP, "implode()" should take the glue parameter first.

Test Plan:
Used the linter introduced in D20857 to identify affected callsites.

$ git grep -i implode | cut -d: -f1 | sort | uniq | xargs arc lint --output summary --never-apply-patches | grep -i glue

Maniphest Tasks: T13428

Differential Revision: https://secure.phabricator.com/D20858