Admin navbar

* contains all of the previous pull request
* puts two new dropdowns in the global navbar: 'Organisations' > 'Without Users, and 'Users' > 'All'
 * these menu options link to their corresponding reports
 * TODO: expand drop down options to include 'Organisations' > 'All', 'Organisations' > 'With Generated Users', and 'Users' > 'Pending Admins'