ENH: UI/UX

[jsheunis]

This PR:

• Display last updated as 'unknown' if the corresponding field is empty (previously an empty value was displayed as an ugly 'NaN')
• For items in the content tab:
  • The cloud icon as a means to download a file is removed
  • filename is turned into a link, if the file has an associated URL and only if the URL string contains http
  • the file size is updated to display correctly (empty string, and not NaN, if the value is empty)
  • the cursor will only turn into a pointer if the item is a folder or a file with a link.
• closes JS displays "Last updated: NaN-0NaN-0NaN" if metadata does not contain source time  #332
• closes ENH: update rendering and behaviour of file downloads #361

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (a75c06b) 86.19% compared to head (498c278) 86.19%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #369   +/-   ##
=======================================
  Coverage   86.19%   86.19%           
=======================================
  Files          43       43           
  Lines        2709     2709           
=======================================
  Hits         2335     2335           
  Misses        374      374           

Flag	Coverage Δ
unittests	79.07% <ø> (ø)

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