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.
* facility name is a link to the facility management page with `Zone = None`

ToDos: 

1. It only displays one Facility on the page (was stuck trying to figure out this bug)
1. Unit tests
1. i18n for messages / strings used specially on the page template

/cc: @jamalex
Our convention is to use underscores in both JS and Python, except for class names, rather than camelCase. Some older code hasn't been converted over yet, but all new code should follow this style.