Doorkeeper oauth error causes an Err500

[blerner]

Followup from #186 - if there's a Doorkeeper authorization error, then it'll start Rendering .gem/ruby/gems/doorkeeper-5.4.0/app/views/doorkeeper/authorizations/error.html.erb within layouts/application. But layouts/application includes the _navbar partial, which attempts to check if true_user != current_user. But true_user isn't even defined, so this causes an ActionView::Template::Error (undefined local variable or method true_user' for #<#Class:0x0000000007e63c30:0x00007f7fa75a3b78>):`, which then barfs the whole page.