Support links in FileExplorer

[mebuie]

File explorer only supports local content. When content exists outside the website, the user has to open a page with the external link. External links should be supported from the FileExplorer.

• Create headers in FileManager module
  • Create function to combine header properties in system.json
• Create formatter function to add link to cell
  • Use ${ replacement string} as replacement string.

[mebuie]

${} can now be used to replace strings in the label column.

Still need a way to combine formatter function with grid properties in system.json.