More aggregation admin work and browse/info pages --> child pages

- Renamed the separate browse/info pages to collectively be 'child pages'
  - Updated all references and class names
  - Updated the .xsd and the reader/writers
  - Renamed Browse_Type_Enum to Visibility_Type_Enum
- Updated admin menu order to rename 'Item Aggregations' --> 'Aggregation Management'
- Added check against system-reserved keywords for aliases and new aggregations
- Portal admins (and curators) can delete empty collections now
- Finished the subcollections tab under collection management / curator screen
- Mostly done with child pages tab under collection management / curator screen
  - User can view list of child pages and add new child pages
  - User can go to a screen to edit a single child page information, and view multiple language versions
  - Soon, curator will be able to add a new version to a single child page