Show geo.log in the Admin area

Adds a tab button on the Admin Logs page (at /admin/logs), to display
geo.log, just like the tab buttons for application.log and others.

See !2845

app/controllers/admin/logs_controller.rb

 class Admin::LogsController < Admin::ApplicationController
+  prepend EE::Admin::LogsController
+
   before_action :loggers
 
   def show

changelogs/unreleased-ee/tc-geo-log-admin-page.yml

+---
+title: Show geo.log in the Admin area
+merge_request: 2845
+author:
+type: added

ee/app/controllers/ee/admin/logs_controller.rb

+module EE::Admin::LogsController
+  def loggers
+    raise NotImplementedError unless defined?(super)
+
+    @loggers ||= super + [
+      Gitlab::GeoLogger
+    ]
+  end
+end

ee/lib/gitlab/geo_logger.rb

+module Gitlab
+  class GeoLogger < Gitlab::Logger
+    def self.file_name_noext
+      'geo'
+    end
+  end
+end

spec/ee/spec/features/admin/admin_browses_log_spec.rb

+require 'spec_helper'
+
+describe 'Admin browses logs' do
+  before do
+    sign_in(create(:admin))
+  end
+
+  it 'shows available log files' do
+    visit admin_logs_path
+
+    expect(page).to have_link 'geo.log'
+  end
+end