Change timezone data structure send to the templates

Change timezone data structure send to the templates

The goal of this is to be able to adapt the timezone form
in template without hacking the HTML already rendered.

  * there are two arrays available:
    * `continents` which contains only a list of available continents
    * `cities` which contains a list of available timezone cities, associated with their continent

Note: there are two distinct array because RainTPL doesn't support nested loop very well.