Bug 19602: Add usage statistics link to administration sidebar menu

Bug 19602: Add usage statistics link to administration sidebar menu

This patch adds a link to the new usage statistics page in the
administration sidebar menu.

To test, apply the patch and view an administration page which includes
the admin sidebar. Confirm that the link to usage statistics is there.

Visit the usage statistics page and confirm that the usage statistics
link in the sidebar is display in bold.

Signed-off-by: Simon Pouchol <simon.pouchol@biblibre.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>