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/controllers/admin/versions_controller.rb

@@ -0,0 +1,58 @@
+module Admin
+  class VersionsController < Admin::BaseController
+    skip_authorization_check
+
+    def index
+      authorize! :index, PaperTrail::Version.new(item_type: 'User')
+      conf_ids_for_organizer = current_user.is_admin? ? Conference.pluck(:id) : Conference.with_role(:organizer, current_user).pluck(:id)
+      @versions = PaperTrail::Version.where(["conference_id IN (?) OR item_type = 'User'", conf_ids_for_organizer])
+    end
+
+    def revert_attribute
+      @version = PaperTrail::Version.find(params[:id])
+      authorize! :revert_attribute, @version
+
+      if params[:attribute] && @version.changeset.reject{ |_, values| values[0].blank? && values[1].blank? }.keys.include?(params[:attribute])
+        if @version.item[params[:attribute]] == @version.changeset[params[:attribute]][0]
+          flash[:error] = 'The item is already in the state that you are trying to revert it back to'
+
+        else
+          @version.item[params[:attribute]] = @version.changeset[params[:attribute]][0]
+          if @version.item.save
+            flash[:notice] = 'The selected change was successfully reverted'
+          else
+            flash[:error] = "An error prohibited this change from being reverted: #{@version.item.errors.full_messages.join('. ')}."
+          end
+        end
+
+      else
+        flash[:error] = 'Revert failed. Attribute missing or invalid'
+      end
+
+      redirect_to admin_revision_history_path
+    end
+
+    def revert_object
+      @version = PaperTrail::Version.find(params[:id])
+      authorize! :revert_object, @version
+
+      if @version.event != 'create'
+        if @version.reify.save
+          flash[:notice] = 'The selected change was successfully reverted'
+        else
+          flash[:error] = "An error prohibited this change from being reverted: #{@version.reify.errors.full_messages.join('. ')}."
+        end
+
+      elsif @version.event == 'create' && @version.item
+        # if @version represets a create event and is not currently deleted
+        @version.item.destroy
+        flash[:notice] = 'The selected change was successfully reverted'
+
+      else
+        flash[:error] = 'The item is already in the state that you are trying to revert it back to'
+      end
+
+      redirect_to admin_revision_history_path
+    end
+  end
+end