title: 'current_user' undefined in rails_admin with clearance
body: <p>I've been using rails_admin v0.7.0 with the clearance gem successfully up this point. I tried to update rails_admin to v1.0 today, but am getting an undefined variable or method error for <code>current_user</code>. In v0.7.0 it appears that <code>RailsAdmin::MainController</code> inherits from <code>ApplicationController</code>, whereas in v1.0 it inherits directly from <code>ActionController::Base</code>, which would explain <code>current_user</code> is now undefined (I believe <code>current_user</code> is defined in <code>ApplicationController</code> with the clearance gem). However, since I'm not finding anyone else with this problem, I'm thinking I must be missing something.</p>
  
  <p>I wasn't the one who set up clearance on this app, but I don't think we're doing anything non-standard with it that would affect this. <code>Clearance::Controller</code> is included in <code>ApplicationController</code>. No special definition of <code>current_user</code>.</p>
  
  <p>config/initializers/rails_admin.rb</p>
  
  <pre><code>RailsAdmin.config do |config|
  
    # Popular gems integration
  
    ## Clearance
    config.authorize_with do |controller|
      unless current_user.admin?
        redirect_to(
          main_app.root_path,
          alert: "You are not permitted to view this page"
        )
      end
    end
  
    config.current_user_method { current_user }
  end
  </code></pre>