Setup basic Revision History page

Sets up paper_trail tracking in important models

and add conference_id to versions for storing conference_id
of conference related objects  as metadata.This will help in querying
for versions related to conferences.

Fix issues with factories & event types initialization.Import paper_trail testing helpers
Fix bug: Creating an event creates a useless version with event update

Add revert and view changes features to revision history

app/views/admin/versions/index.html.haml

@@ -0,0 +1,77 @@
+.row
+  .col-md-12
+    .page-header
+      %h1 Revision History
+      %p.text-muted
+        Log of changes made to conferences and associated resources
+
+%table.table.table-striped.table-bordered.table-hover#versionstable
+  %thead
+    %td ID
+    %td Description
+    %td Actions
+  %tbody
+    - @versions.each do |version|
+      %tr
+        %td.col-md-1
+          = version.id
+        %td.col-md-9
+          %p
+            = change_creator_link(version.whodunnit) unless version.item_type == 'User'
+
+            - case version.item_type
+            - when 'Event'
+              = event_change_description(version)
+
+            -when 'UsersRole'
+              = users_role_change_description(version)
+
+            - when 'Subscription'
+              = subscription_change_description(version)
+
+            - when 'Registration', 'EventsRegistration'
+              = registration_change_description(version)
+
+            - when 'Comment'
+              = comment_change_description(version)
+
+            - when 'Vote'
+              = vote_change_description(version)
+
+            - when 'User'
+              = user_change_description(version)
+
+            - else
+              - if version.event == 'create'
+                = 'created new'
+              - elsif version.event == 'update'
+                = "updated #{updated_attributes(version)} of"
+              - else
+                = 'deleted'
+
+            =  render partial: 'object_desc_and_link', locals: { version: version }
+
+            %small.text-muted
+              = distance_of_time_in_words(Time.now,version.created_at) + ' ago'
+              %br
+              = "(#{version.created_at.strftime('%B %-d, %Y %H:%M')})"
+
+          %br
+          = render partial: 'object_changes', locals: { version: version }
+
+        %td.col-md-2
+          .btn-group{role: 'group'}
+            %a.btn.btn-success.btn-sm.show-changeset{id: version.id} View Changes
+            - if can? :revert_object, version
+              %button.btn.btn-default.dropdown-toggle.btn-sm.btn-primary{'data-toggle' => 'dropdown', type: 'button'}
+                Revert
+                %span.caret
+              %ul.dropdown-menu
+                %li= link_to 'All Changes', admin_revision_history_revert_object_path(id: version.id), data: { confirm: 'Are you sure you want to revert this change?' }
+
+                - if can? :revert_attribute, version
+                  %li.divider{role: 'separator'}
+                  - version.changeset.reject{ |_, values| values[0].blank? && values[1].blank? }.each do |attribute, values|
+                    %li= link_to attribute, admin_revision_history_revert_attribute_path(id: version.id, attribute: attribute), data: { confirm: "Are you sure you want to revert #{attribute}?" }
+            - else
+              %button.btn.btn-sm.btn-primary.disabled Revert