Implement Health Status view, fixes #306

Initial PR.

To follow up:
- list localizers with https://github.com/pascalchevrel/list_localizers (per repo/per year, and globally for the locale) And maybe globally for all locales
- Improve view performances (cache)
- Fix getStatsPaneDP test (currently disabled)
- Shall we remove code for http://transvision.mozfr.org/showrepos/ or both views should coexist?
- Last commit is most of the times from L10n Drivers, even on Desktop
- Shall we merge in the UI some of the components on Desktop?
- Prettify UI
Personally, if you want to keep lines short, I find this code style easier to read.
```
$tabs .= self::getStatsTab(
    $stats['stats'],
    'sub-' . $stats['stats']['repo'], 
    $first_tab
);
```