Implemented EZP-19948 - Global variable available in twig

The point of this PR is to expose *global* information in a helper service, available from any template, just as it was possible in eZ Publish 4.x with [variables in pagelayout](http://doc.ez.no/eZ-Publish/Technical-manual/4.x/Templates/The-pagelayout/Variables-in-pagelayout).

The legacy part is meant to increase backward compatibility with 4.x and ease migration to 5.x.

## TODO

- [x] Expose the helper as an **ezpublish** Twig global variable (helper is a service)
- [x] Expose base information (*siteaccess*, *requestedUriString*, *systemUriString*, *viewParameters*)
- [x] Expose config resolver
- [x] Add a *legacy* property ( **ezpublish.legacy** ) from *EzPublishLegacyBundle*
- [x] Expose base *legacy* information, when available (legacy kernel booted) : *ezinfo*, *navigation_part*, *ui_component*, *ui_context*
- [x] Expose *legacy* persistent variable
- [x] Expose *legacy* CSS/JS resources requested through `ezscript_require()` and `ezcss_require()`
- [x] Check information from `ezpagedata()`
- [x] Tests