Make unlisted projects

It will be nice to have unlisted projects excluded from search and profile's listing, but available for users with link.

I believe that we need to add the following type of access restrictions:

• Project is only accessible by URL
• Even unauthenticated users could access the project

Here are some usecases:

• I have have a project that is used to host RST documentation. I have wrote those documentation for another company and would like to share it unconditionally with anyone who has a link. It will not be an issue of this documentation will be accessed by someone, but I don't want to share it publicly.
• I use gitlab to host my CV (another REST doc) and it would be nice if I could share it with a simple link.
• Some projects may have use this feature to share the code with a subset of users.
• It wasn't mine idea, but I agree that it will be great to share the code examples, completed tasks and stuff like this.