Show severity in Shiny app

[MaximMoinat]

First test, retrieves severity from checkDescription upon opening shiny app and shows it in the check results table.

Next step is to add a table which shows counts fails by severity.

Closes #657

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.14%. Comparing base (5609499) to head (312047d).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #658   +/-   ##
========================================
  Coverage    87.14%   87.14%           
========================================
  Files           17       17           
  Lines         1066     1066           
========================================
  Hits           929      929           
  Misses         137      137           

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