Remove New project in this group button from main nav

What is the problem we are trying to solve?

If you have Private checked under the Restricted visibility levels field in the admin area then non-admins can not create projects in private groups.

Proposal to solve this problem

When Private is checked under the Restricted visibility levels setting and the user is in a private group, the New project button in the main nav should not be shown.

Designs

Remove button outlined in red

Engineering implementation plan

This issue and #384355 (closed) may be able to be solved at the same time.

• Update permissions check in app/helpers/nav/new_dropdown_helper.rb#L29