Add 'Created on' column

[matthew-matvei]

Feature

Following the incomplete implementation of #3, add a Created on column along with other column headers.

• A new 'Created on' column appears in the directory header
• Dates in the created / modified on columns formatted to save space (e.g., '23:59:59, 30/12/2018')

Proposed solution

• Extending columns should be straight forward, particularly as long as there is enough room to show four columns (some re-proportioning of columns will be necessary
  • This will also involve further nesting of the <SplitPane />s in the <DirectoryHeader /> component
• Formatting the dates should be simple following moment formatting documentation