Timeline Authoring Bug Fixes

Code review approved at http://mrccodereview.cloudapp.net/ui#review:id=1158

This addresses two issues tracked by:
1. https://github.com/alterm4nn/ChronoZoom/issues/242
Child timeline's start and end dates should lie within the start and end dates of their parent timeline.

2. https://github.com/alterm4nn/ChronoZoom/issues/336
The default parent timeline for a new timeline that is created by any user defaults to the Cosmos timeline. It should default to null so timelines don't cross user boundaries + root timelines are allowed in collections other that the Beta collection.