Hide directories beginning with _ from the dashboard.

Closes gh-5152
Case insensitive sorting in the dashboard.

Closes gh-5151
Don't attempt locale-aware sorting for now.

On Python 2, locale.strxfrm() can't handle unicode.
Make hidden directories configurable
Fix a couple of static analysis warnings
Merge pull request #5153 from takluyver/dashboard-sorting

Dashboard sorting

closes #5151
closes #5152