R1:1db4de7dbc2c

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…