Add overview of active sessions (#3929)

* Add overview of active sessions

* Better display of browser/platform name

* Improve how browser information is stored and displayed for sessions overview

* Fix test

db/migrate/20170624134742_add_description_to_session_activations.rb

@@ -0,0 +1,7 @@
+class AddDescriptionToSessionActivations < ActiveRecord::Migration[5.1]
+  def change
+    add_column :session_activations, :user_agent, :string, null: false, default: ''
+    add_column :session_activations, :ip, :inet
+    add_foreign_key :session_activations, :users, on_delete: :cascade
+  end
+end