Export csv form and datepicker logic and css.

This PR targets issue #1231 

Added button to show/hide csv export form;
Added script src to central base.html;
Changed the logic (js) related to the datepicker to set the dates in the form;
Added css hover effect to both buttons;

The following files were changed:
- kalite/static/css/control_panel.css
- kalite/templates/central/base.html
- kalite/templates/control_panel/base.html
- kalite/templates/control_panel/facility_usage.html