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
clear _reply_content cache before using it

Any exceptions raised in startup code could be interpreted
as the first actual execution failing.
Merge pull request #5242 from minrk/no-abort-startup

clear _reply_content cache before using it. This could cause startup exceptions to abort all-cell notebook execution, for example.
allow custom headers on all pages
Merge pull request #5169 from Carreau/allow-cors

Allow custom headers in tornado
Merge pull request #5153 from takluyver/dashboard-sorting

Dashboard sorting

closes #5151
closes #5152
make one strptime call at import of jsonutil

avoids thread safety issues (Python bug #7980)
Merge pull request #5252 from minrk/strptime-not-threadsafe

make one strptime call at import of jsonutil
Refactor to allow relative/full module paths
ENH: Enable custom tornado handlers via config