Improve database properties view: (#501)

- Compatibility Level should be a numerical value. In using `enum.ToString()` the result was `Version140` instead of `140`
- Backup date should be sensible if never backed up. Added a simple check that states it was never backed up if the backup date is before 1900.

src/Microsoft.SqlTools.ServiceLayer/Localization/sr.strings

@@ -802,6 +802,7 @@ general_containmentType_None = None
 general_containmentType_Partial = Partial
 filegroups_filestreamFiles = FILESTREAM Files
 prototype_file_noApplicableFileGroup = No Applicable Filegroup
+NeverBackedUp = Never
 
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 # Backup Service