re-assigned to the correct record.

* Renamed build to create since the methods now create objects.
* Removed call to Project#save that isn't needed since objects are saved directly now.
* Wrapped the activity creation and updates in a SQL transaction so TimeEntries
  will remain in a consistent state if there is an error.
* When a Project's TimeEntryActivities are destroyed, they are now
  reassigned to the parent TimeEntryActivity.

  #4077

git-svn-id: https://svn.redmine.org/redmine/trunk@2950 e93f8b46-1217-0410-a6f0-8f06a7374b81