Add information dialogs to adding project members if unsupported

Summary: If you try to join or add members to a parent project, we currently return 404. This instead adds an informational dialog. Fixes T11055

Test Plan: Click on Join Project and Add Members while on a Parent Project or Milestone.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11055

Differential Revision: https://secure.phabricator.com/D16000