Admin (non-superuser) can reduce own access - noob user error #967

Under "Manage Access", an admin user can reduce his own access for a site from Admin to View or No Access. This does not generate a pop-up confirmation. Trying to click on "Admin" has no effect because Piwik has re-auth'd the user. Refreshing the page takes the user to the Login form.

In [1605], fixes #967 - admin (non-superuser) can reduce access when single site selected; this is now consistent with the behaviour when 'all sites' is selected