Send emails via sidekiq (#380)

* add sidekiq web view if the user is an administrator

* add sidekiq setup configuration and support

* update devise to use delivery_later method and update test

* update conversation to use deliver_later instead of deliver

* Update Routes

* Add Procfile for Heroku One-Click Start

* updating docs

* update concurrency and Procfile for supporting Heroku Free Dyno

* update Procfile.dev

config/sidekiq.yml

@@ -0,0 +1,23 @@
+# Sample configuration file for Sidekiq.
+# Options here can still be overridden by cmd line args.
+# Place this file at config/sidekiq.yml and Sidekiq will
+# pick it up automatically.
+---
+:verbose: false
+:concurrency: 10
+:timeout: 25
+
+# Sidekiq will run this file through ERB when reading it so you can
+# even put in dynamic logic, like a host-specific queue.
+# http://www.mikeperham.com/2013/11/13/advanced-sidekiq-host-specific-queues/
+:queues:
+  - critical
+  - default
+  - low
+  - mailers
+
+# you can override concurrency based on environment
+production:
+  :concurrency: 3
+staging:
+  :concurrency: 15