Add option to disable project self-association

[themightychris]

Currently, anyone can add someone else or themselves as a member of a project.

Add a new configuration option at the site level that restricts adding new members to a project to site staff and existing project members

[themightychris]

Or maybe a three-way setting:

• public: any registered user can add members to a project
• members: any member of a project can add members
• leadership: only maintainers of projects can add members

In all three cases, site-level staff/admins would still be able to add members

[machow]

This makes a lot of sense to me--thanks for creating! I would say for me, the biggest motivation is the first bullet (restricting public users), just to avoid creating confusion around who is organizing the project.