Admin menu improvements and made internal name mandatory

[adam-murray]

• Added Internal name, and modified date fields

• Made Internal name a mandatory field

• Added Internal name, URL and modified date to list_display (in that order)

• Sort by Internal name (A-Z) by default and made all other columns sortable by

• Enabled search for both the Internal name and URL fields

  • Created filters for Site and Type

  • Keep Relative path selected by default in Type