Fix smushing of multiple values in Projects "Additional Hashtags" field

Summary: Ref T10168. When we render this control, we currently don't put commas into the value correctly if there are multiple alternative hashtags.

Test Plan: Edited a project with multiple alternate hashtags. Before change: they all got smushed together. After change: properly comma-separated.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10168

Differential Revision: https://secure.phabricator.com/D15045