FEATURE: allow plugins to easily detect if running in Rack

Usage: Discourse.running_in_rack? to tell if rack was booted

config.ru

@@ -1,5 +1,8 @@
 # This file is used by Rack-based servers to start the application.
+ENV["DISCOURSE_RUNNING_IN_RACK"] = "1"
+
 require ::File.expand_path('../config/environment',  __FILE__)
+
 map ActionController::Base.config.try(:relative_url_root) || "/" do
   run Discourse::Application
 end

lib/discourse.rb

@@ -499,4 +499,8 @@ module Discourse
     nil
   end
 
+  def self.running_in_rack?
+    ENV["DISCOURSE_RUNNING_IN_RACK"] == "1"
+  end
+
 end

spec/components/discourse_spec.rb

@@ -15,6 +15,18 @@ describe Discourse do
 
   end
 
+  context 'running_in_rack' do
+    after do
+      ENV.delete("DISCOURSE_RUNNING_IN_RACK")
+    end
+
+    it 'should not be running in rack' do
+      expect(Discourse.running_in_rack?).to eq(false)
+      ENV["DISCOURSE_RUNNING_IN_RACK"] = "1"
+      expect(Discourse.running_in_rack?).to eq(true)
+    end
+  end
+
   context 'base_url' do
     context 'when https is off' do
       before do