Add ability to enable private projects to a selected list of users

[bugzilla-to-github]

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Bug 1627878

Bug Reporter: @mathjazz CC: [email protected] Depends on: Bug 1627877

This feature implements the ability to show projects only to a selected list of users: https://github.com/mozilla/pontoon/blob/master/specs/0100-private-projects.md

It allows an organization to have specific localizers working on specific projects. This requires having a configuration option to select a list of users who will be able to see, and interact, with that project.