Draft: Handle moderation privileges

Fix #206 (closed)
For BE MR pleroma!3676 (closed)

• Make admin-fe aware of privileges
• Make sure a privileged non-admin can log in
• Only show menu/options to those who are privileged
  • Sidebar
  • Make sure you're redirected to a privileged route after login
  • Only show privileged options in the pages themselves
    • Users
      • "Moderate multiple users" dropdown
        • Bug: Changing tags wont show if the user can't fetch the config (i.e. is non-admin)
      • "Moderation" dropdown
      • "Create new account"
      • Click a user > "Moderate user"
    • Statuses
      • "Change scope" dropdown
    • Reports
      • "Moderate user" dropdown
    • Moderation Log
      • Link to users and to specific reports
  • Don't do unprivileged calls (e.g. after login /api/pleroma/admin/config is called)
• Make sure error messages are correct (e.g. when not logged in, you now get message about needing to be admin, but not only admins are able to log in)
• What if I have privileges, but none to give me a menu? What happens then, is that acceptable, and how to handle it properly? => You're now redirected to the /401 page.
• Check if #213 could/should be done here