Feature/2311 advance to the next unit functionality

Hi @aronasorman.  This is for #2311:
* url for the page is at http://127.0.0.1:8008/test/current_unit/
* maximum number of units are set to 20 for now but can easily be changed either on `settings.py` or using `Settings` model.
* user can click on `Previous` or `Next` buttons to set the current unit for the facility or select the number shown as link.

ToDos: 
* It only displays one Facility on the page.
* Unit tests

/cc: @jamalex