Provide "Initial Members" instead of default joining projects

Summary:
Ref T10010. Instead of autojoining projects, provide "Initial Members: [___]" that the user can fill in.

This is only available in the web UI when creating a (non-milestone) project.

Test Plan:

• Created a new project with no members.
• Created a new project with some members.
• Created a new milestone (no control).
• Created a new project with myself as a member and an "Editable By: Project Members" policy, to verify this use case still works properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D15195