Don't show "Primary Hashtag" when creating a project

Summary: Fixes T6598, "Primary Hashtag" field should only be visible in edit mode of existing projects.

Test Plan: Create project, "Primary Hashtag" field should be hidden. Edit an existing project, "Primary Hashtag" field should appear above "Additional Hashtags" as before.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T6598

Differential Revision: https://secure.phabricator.com/D10964