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
Please use the Handlebars templating system we have in place; I just put up a Wiki page with some details:
https://github.com/learningequality/ka-lite/wiki/Approach-to-client-side-templating-(Handlebars.js)