When an issuable's state changes, or one is created, we should clear the cache
counts for a user's assigned issuables, and also the project-wide caches for
this user type.
This runs a slightly slower query to get the issue and MR counts in the
navigation, but caches by user type (can see all / none confidential issues) for
two minutes.
When we are filtering by a single project, and the current user has access to
see confidential issues on that project, we don't need to filter by
confidentiality at all - just as if the user were an admin.
The filter by confidentiality often picks a non-optimal query plan: for
instance, AND-ing the results of all issues in the project (a relatively small
set), and all issues in the states requested (a huge set), rather than just
starting small and winnowing further.
In CE only the admin has access to all private groups & projects. In EE also an
auditor can have full private access.
To overcome merge conflicts, or accidental incorrect access rights, abstract
this out in `User#full_private_access?`.
`User#admin?` now only should be used for admin-only features. For private
access-related features `User#full_private_access?` should be used.
Backported from gitlab-org/gitlab-ee!2199
