Add possibility to make projects visible only to by some users

Add a way to declare some projects "private" so that they are visible ONLY to some users. The project name should not be visible to the other users.

For instance, if I create a "public JIRA" to allow my customers to enter issues, I don't want the other customers to know about the project (in the project list for instance). Only some users should be allowed (explicitly) to see this project.

Once the project is displayed, other permissions apply.